diff --git a/extern/Base64/Base64.h b/extern/Base64/Base64.h
index cdfdc04d0b..498d2be128 100644
--- a/extern/Base64/Base64.h
+++ b/extern/Base64/Base64.h
@@ -48,7 +48,7 @@ class Base64 {
     size_t out_len = 4 * ((in_len + 2) / 3);
     std::string ret(out_len, '\0');
     size_t i;
-    char *p = const_cast<char*>(ret.c_str());
+    char *p = ret.data();
 
     for (i = 0; i < in_len - 2; i += 3) {
       *p++ = sEncodingTable[(data[i] >> 2) & 0x3F];