diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a24367c8..a4f85ea2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -86,6 +86,12 @@ jobs:
         command:  check
         args: --features attributes
 
+    - name: build unstable only
+      uses: actions-rs/cargo@v1
+      with:
+        command:  build
+        args: --no-default-features --features unstable
+
     - name: tests
       uses: actions-rs/cargo@v1
       with:
diff --git a/src/os/windows/mod.rs b/src/os/windows/mod.rs
index 6dac11b6..67bf0372 100644
--- a/src/os/windows/mod.rs
+++ b/src/os/windows/mod.rs
@@ -5,5 +5,6 @@ cfg_std! {
 }
 
 cfg_unstable! {
+    #[cfg(feature = "default")]
     pub mod fs;
 }
diff --git a/src/utils.rs b/src/utils.rs
index 31290e33..db916ed6 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -66,7 +66,7 @@ mod timer {
 
 #[cfg(any(feature = "unstable", feature = "default"))]
 pub(crate) fn timer_after(dur: std::time::Duration) -> timer::Timer {
-    #[cfg(not(target_os = "unknown"))]
+    #[cfg(all(not(target_os = "unknown"), feature = "default"))]
     once_cell::sync::Lazy::force(&crate::rt::RUNTIME);
 
     Timer::after(dur)