diff --git a/libc/src/assert.cpp b/libc/src/assert.cpp
index f24110b6..c447945e 100644
--- a/libc/src/assert.cpp
+++ b/libc/src/assert.cpp
@@ -7,8 +7,7 @@ extern "C"
 {
     _weak [[noreturn]] void __assertion_failed(const char* file, int line, const char* function, const char* expr)
     {
-        // FIXME: Output to standard error instead of standard output.
-        printf("%s:%d: %s: Assertion '%s' failed.\n", file, line, function, expr);
+        fprintf(stderr, "%s:%d: %s: Assertion '%s' failed.\n", file, line, function, expr);
         abort();
     }
 }
diff --git a/libluna/src/Check.cpp b/libluna/src/Check.cpp
index b4d1bcb4..edd1df74 100644
--- a/libluna/src/Check.cpp
+++ b/libluna/src/Check.cpp
@@ -6,7 +6,6 @@
 
 _weak [[noreturn]] bool __check_failed(SourceLocation location, const char* expr)
 {
-    // FIXME: Output to standard error instead of standard output.
-    printf("Check failed at %s:%d in %s: %s\n", location.file(), location.line(), location.function(), expr);
+    fprintf(stderr, "Check failed at %s:%d in %s: %s\n", location.file(), location.line(), location.function(), expr);
     abort();
 }