--- a/xpcom/tests/TestBlockingProcess.cpp
+++ b/xpcom/tests/TestBlockingProcess.cpp
@@ -1,8 +1,9 @@
 #include <stdio.h>
+#include "mozilla/Unused.h"
 
 int main()
 {
   char tmp;
-  fread(&tmp, sizeof(tmp), 1, stdin);
+  mozilla::Unused << fread(&tmp, sizeof(tmp), 1, stdin);
   return 0;
 }