diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0d61907f46..1ee8bd8d92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -75,6 +75,7 @@
     Bug #6363: Some scripts in Morrowland fail to work
     Bug #6376: Creatures should be able to use torches
     Bug #6386: Artifacts in water reflection due to imprecise screen-space coordinate computation
+    Bug #6396: Inputting certain Unicode characters triggers an assertion
     Bug #6416: Morphs are applied to the wrong target
     Feature #890: OpenMW-CS: Column filtering
     Feature #2554: Modifying an object triggers the instances table to scroll to the corresponding record
diff --git a/apps/niftest/niftest.cpp b/apps/niftest/niftest.cpp
index cb6205ef5c..c42df4a423 100644
--- a/apps/niftest/niftest.cpp
+++ b/apps/niftest/niftest.cpp
@@ -2,8 +2,8 @@
 
 #include <iostream>
 #include <fstream>
-#include <cstdlib>
 
+#include <components/misc/stringops.hpp>
 #include <components/nif/niffile.hpp>
 #include <components/files/constrainedfilestream.hpp>
 #include <components/vfs/manager.hpp>
@@ -18,18 +18,10 @@ namespace bpo = boost::program_options;
 namespace bfs = boost::filesystem;
 
 ///See if the file has the named extension
-bool hasExtension(std::string filename, std::string  extensionToFind)
+bool hasExtension(std::string filename, std::string extensionToFind)
 {
     std::string extension = filename.substr(filename.find_last_of('.')+1);
-
-    //Convert strings to lower case for comparison
-    std::transform(extension.begin(), extension.end(), extension.begin(), ::tolower);
-    std::transform(extensionToFind.begin(), extensionToFind.end(), extensionToFind.begin(), ::tolower);
-
-    if(extension == extensionToFind)
-        return true;
-    else
-        return false;
+    return Misc::StringUtils::ciEqual(extension, extensionToFind);
 }
 
 ///See if the file has the "nif" extension.
diff --git a/apps/opencs/model/filter/parser.cpp b/apps/opencs/model/filter/parser.cpp
index d363b4849d..6f185d60fa 100644
--- a/apps/opencs/model/filter/parser.cpp
+++ b/apps/opencs/model/filter/parser.cpp
@@ -17,6 +17,19 @@
 #include "textnode.hpp"
 #include "valuenode.hpp"
 
+namespace
+{
+    bool isAlpha(char c)
+    {
+        return std::isalpha(static_cast<unsigned char>(c));
+    }
+
+    bool isDigit(char c)
+    {
+        return std::isdigit(static_cast<unsigned char>(c));
+    }
+}
+
 namespace CSMFilter
 {
     struct Token
@@ -103,7 +116,7 @@ CSMFilter::Token CSMFilter::Parser::getStringToken()
     {
         char c = mInput[mIndex];
 
-        if (std::isalpha (c) || c==':' || c=='_' || (!string.empty() && std::isdigit (c)) || c=='"' ||
+        if (isAlpha(c) || c==':' || c=='_' || (!string.empty() && isDigit(c)) || c=='"' ||
             (!string.empty() && string[0]=='"'))
             string += c;
         else
@@ -150,7 +163,7 @@ CSMFilter::Token CSMFilter::Parser::getNumberToken()
     {
         char c = mInput[mIndex];
 
-        if (std::isdigit (c))
+        if (isDigit(c))
         {
             string += c;
             hasDigit = true;
@@ -225,10 +238,10 @@ CSMFilter::Token CSMFilter::Parser::getNextToken()
         case '!': ++mIndex; return Token (Token::Type_OneShot);
     }
 
-    if (c=='"' || c=='_' || std::isalpha (c) || c==':')
+    if (c=='"' || c=='_' || isAlpha(c) || c==':')
         return getStringToken();
 
-    if (c=='-' || c=='.' || std::isdigit (c))
+    if (c=='-' || c=='.' || isDigit(c))
         return getNumberToken();
 
     error();
diff --git a/apps/opencs/model/world/idtable.cpp b/apps/opencs/model/world/idtable.cpp
index 5b4a9b31bc..215f42133d 100644
--- a/apps/opencs/model/world/idtable.cpp
+++ b/apps/opencs/model/world/idtable.cpp
@@ -8,6 +8,7 @@
 #include <stdexcept>
 
 #include <components/esm/cellid.hpp>
+#include <components/misc/stringops.hpp>
 
 #include "collectionbase.hpp"
 #include "columnbase.hpp"
@@ -354,8 +355,7 @@ CSMWorld::LandTextureIdTable::ImportResults CSMWorld::LandTextureIdTable::import
     for (int i = 0; i < idCollection()->getSize(); ++i)
     {
         auto& record = static_cast<const Record<LandTexture>&>(idCollection()->getRecord(i));
-        std::string texture = record.get().mTexture;
-        std::transform(texture.begin(), texture.end(), texture.begin(), tolower);
+        std::string texture = Misc::StringUtils::lowerCase(record.get().mTexture);
         if (record.isModified())
             reverseLookupMap.emplace(texture, idCollection()->getId(i));
     }
@@ -376,8 +376,7 @@ CSMWorld::LandTextureIdTable::ImportResults CSMWorld::LandTextureIdTable::import
 
         // Look for a pre-existing record
         auto& record = static_cast<const Record<LandTexture>&>(idCollection()->getRecord(oldRow));
-        std::string texture = record.get().mTexture;
-        std::transform(texture.begin(), texture.end(), texture.begin(), tolower);
+        std::string texture = Misc::StringUtils::lowerCase(record.get().mTexture);
         auto searchIt = reverseLookupMap.find(texture);
         if (searchIt != reverseLookupMap.end())
         {
diff --git a/apps/openmw/mwinput/keyboardmanager.cpp b/apps/openmw/mwinput/keyboardmanager.cpp
index b8019b12ba..b121b665e7 100644
--- a/apps/openmw/mwinput/keyboardmanager.cpp
+++ b/apps/openmw/mwinput/keyboardmanager.cpp
@@ -42,9 +42,9 @@ namespace MWInput
 
         bool consumed = SDL_IsTextInputActive() &&  // Little trick to check if key is printable
                         (!(SDLK_SCANCODE_MASK & arg.keysym.sym) &&
-                        (std::isprint(arg.keysym.sym) ||
                         // Don't trust isprint for symbols outside the extended ASCII range
-                        (kc == MyGUI::KeyCode::None && arg.keysym.sym > 0xff)));
+                        ((kc == MyGUI::KeyCode::None && arg.keysym.sym > 0xff) ||
+                        (arg.keysym.sym >= 0 && arg.keysym.sym <= 255 && std::isprint(arg.keysym.sym))));
         if (kc != MyGUI::KeyCode::None && !mBindingsManager->isDetectingBindingState())
         {
             if (MWBase::Environment::get().getWindowManager()->injectKeyPress(kc, 0, arg.repeat))
diff --git a/apps/openmw/mwstate/character.cpp b/apps/openmw/mwstate/character.cpp
index 59ddd2cd10..52696de104 100644
--- a/apps/openmw/mwstate/character.cpp
+++ b/apps/openmw/mwstate/character.cpp
@@ -8,6 +8,8 @@
 #include <components/esm/esmreader.hpp>
 #include <components/esm/defs.hpp>
 
+#include <components/misc/utf8stream.hpp>
+
 bool MWState::operator< (const Slot& left, const Slot& right)
 {
     return left.mTimeStamp<right.mTimeStamp;
@@ -52,12 +54,14 @@ void MWState::Character::addSlot (const ESM::SavedGame& profile)
     std::ostringstream stream;
 
     // The profile description is user-supplied, so we need to escape the path
-    for (std::string::const_iterator it = profile.mDescription.begin(); it != profile.mDescription.end(); ++it)
+    Utf8Stream description(profile.mDescription);
+    while(!description.eof())
     {
-        if (std::isalnum(*it))  // Ignores multibyte characters and non alphanumeric characters
-            stream << *it;
+        auto c = description.consume();
+        if(c <= 0x7F && std::isalnum(c)) // Ignore multibyte characters and non alphanumeric characters
+            stream << static_cast<char>(c);
         else
-            stream << "_";
+            stream << '_';
     }
 
     const std::string ext = ".omwsave";
diff --git a/apps/openmw/mwstate/charactermanager.cpp b/apps/openmw/mwstate/charactermanager.cpp
index 027a4f38a4..301f33c5df 100644
--- a/apps/openmw/mwstate/charactermanager.cpp
+++ b/apps/openmw/mwstate/charactermanager.cpp
@@ -5,6 +5,8 @@
 
 #include <boost/filesystem.hpp>
 
+#include <components/misc/utf8stream.hpp>
+
 MWState::CharacterManager::CharacterManager (const boost::filesystem::path& saves,
     const std::vector<std::string>& contentFiles)
 : mPath (saves), mCurrent (nullptr), mGame (getFirstGameFile(contentFiles))
@@ -57,12 +59,14 @@ MWState::Character* MWState::CharacterManager::createCharacter(const std::string
     std::ostringstream stream;
 
     // The character name is user-supplied, so we need to escape the path
-    for (std::string::const_iterator it = name.begin(); it != name.end(); ++it)
+    Utf8Stream nameStream(name);
+    while(!nameStream.eof())
     {
-        if (std::isalnum(*it)) // Ignores multibyte characters and non alphanumeric characters
-            stream << *it;
+        auto c = nameStream.consume();
+        if(c <= 0x7F && std::isalnum(c)) // Ignore multibyte characters and non alphanumeric characters
+            stream << static_cast<char>(c);
         else
-            stream << "_";
+            stream << '_';
     }
 
     boost::filesystem::path path = mPath / stream.str();
diff --git a/components/files/escape.hpp b/components/files/escape.hpp
index d01bd8d980..310042072d 100644
--- a/components/files/escape.hpp
+++ b/components/files/escape.hpp
@@ -84,7 +84,7 @@ namespace Files
             {
                 mNext.push(character);
             }
-            if (!mSeenNonWhitespace && !isspace(character))
+            if (!mSeenNonWhitespace && !(character >= 0 && character <= 255 && isspace(character)))
                 mSeenNonWhitespace = true;
         }
         int retval = mNext.front();
diff --git a/components/interpreter/defines.cpp b/components/interpreter/defines.cpp
index 48d2ceaa45..164e7b2126 100644
--- a/components/interpreter/defines.cpp
+++ b/components/interpreter/defines.cpp
@@ -172,8 +172,7 @@ namespace Interpreter{
 
                         for(unsigned int j = 0; j < globals.size(); j++){
                             if(globals[j].length() > temp.length()){ // Just in case there's a global with a huuuge name
-                                temp = text.substr(i+1, globals[j].length());
-                                transform(temp.begin(), temp.end(), temp.begin(), ::tolower);
+                                temp = Misc::StringUtils::lowerCase(text.substr(i+1, globals[j].length()));
                             }
 
                             found = check(temp, globals[j], &i, &start);
diff --git a/components/lua/configuration.cpp b/components/lua/configuration.cpp
index 4598ed2508..172d958029 100644
--- a/components/lua/configuration.cpp
+++ b/components/lua/configuration.cpp
@@ -30,6 +30,11 @@ namespace LuaUtil
             {"POTION", ESM::LuaScriptCfg::sPotion},
             {"WEAPON", ESM::LuaScriptCfg::sWeapon},
         };
+
+        bool isSpace(char c)
+        {
+            return std::isspace(static_cast<unsigned char>(c));
+        }
     }
 
     const std::vector<int> ScriptsConfiguration::sEmpty;
@@ -101,11 +106,11 @@ namespace LuaUtil
             if (!line.empty() && line.back() == '\r')
                 line = line.substr(0, line.size() - 1);
 
-            while (!line.empty() && std::isspace(line[0]))
+            while (!line.empty() && isSpace(line[0]))
                 line = line.substr(1);
             if (line.empty() || line[0] == '#')  // Skip empty lines and comments
                 continue;
-            while (!line.empty() && std::isspace(line.back()))
+            while (!line.empty() && isSpace(line.back()))
                 line = line.substr(0, line.size() - 1);
 
             if (!Misc::StringUtils::ciEndsWith(line, ".lua"))
@@ -118,7 +123,7 @@ namespace LuaUtil
                 throw std::runtime_error(Misc::StringUtils::format("No flags found in: %s", std::string(line)));
             std::string_view flagsStr = line.substr(0, semicolonPos);
             std::string_view scriptPath = line.substr(semicolonPos + 1);
-            while (std::isspace(scriptPath[0]))
+            while (isSpace(scriptPath[0]))
                 scriptPath = scriptPath.substr(1);
 
             // Parse flags
@@ -126,10 +131,10 @@ namespace LuaUtil
             size_t flagsPos = 0;
             while (true)
             {
-                while (flagsPos < flagsStr.size() && (std::isspace(flagsStr[flagsPos]) || flagsStr[flagsPos] == ','))
+                while (flagsPos < flagsStr.size() && (isSpace(flagsStr[flagsPos]) || flagsStr[flagsPos] == ','))
                     flagsPos++;
                 size_t startPos = flagsPos;
-                while (flagsPos < flagsStr.size() && !std::isspace(flagsStr[flagsPos]) && flagsStr[flagsPos] != ',')
+                while (flagsPos < flagsStr.size() && !isSpace(flagsStr[flagsPos]) && flagsStr[flagsPos] != ',')
                     flagsPos++;
                 if (startPos == flagsPos)
                     break;
diff --git a/components/misc/stringops.hpp b/components/misc/stringops.hpp
index 12633db826..fe4be00006 100644
--- a/components/misc/stringops.hpp
+++ b/components/misc/stringops.hpp
@@ -184,17 +184,16 @@ public:
 
     static inline void trim(std::string &s)
     {
-        // left trim
-        s.erase(s.begin(), std::find_if(s.begin(), s.end(), [](int ch)
+        const auto notSpace = [](char ch)
         {
-            return !std::isspace(ch);
-        }));
+            // TODO Do we care about multibyte whitespace?
+            return !std::isspace(static_cast<unsigned char>(ch));
+        };
+        // left trim
+        s.erase(s.begin(), std::find_if(s.begin(), s.end(), notSpace));
 
         // right trim
-        s.erase(std::find_if(s.rbegin(), s.rend(), [](int ch)
-        {
-            return !std::isspace(ch);
-        }).base(), s.end());
+        s.erase(std::find_if(s.rbegin(), s.rend(), notSpace).base(), s.end());
     }
 
     template <class Container>
diff --git a/components/misc/utf8stream.hpp b/components/misc/utf8stream.hpp
index 9dc8aa8208..8435816c67 100644
--- a/components/misc/utf8stream.hpp
+++ b/components/misc/utf8stream.hpp
@@ -3,6 +3,7 @@
 
 #include <cstring>
 #include <string>
+#include <string_view>
 #include <tuple>
 
 class Utf8Stream
@@ -30,6 +31,11 @@ public:
     {
     }
 
+    Utf8Stream (std::string_view str) :
+        Utf8Stream (reinterpret_cast<Point>(str.data()), reinterpret_cast<Point>(str.data() + str.size()))
+    {
+    }
+
     bool eof () const
     {
         return cur == end;