diff --git a/js/src/assembler/wtf/Assertions.h b/js/src/assembler/wtf/Assertions.h
index 1c1d63674e8c..29a4f10f7e59 100644
--- a/js/src/assembler/wtf/Assertions.h
+++ b/js/src/assembler/wtf/Assertions.h
@@ -38,7 +38,10 @@
 #endif
 
 #define ASSERT(assertion) MOZ_ASSERT(assertion)
-#define ASSERT_UNUSED(variable, assertion) (((void)variable), ASSERT(assertion))
+#define ASSERT_UNUSED(variable, assertion) do { \
+    (void)variable; \
+    ASSERT(assertion); \
+} while (0)
 #define ASSERT_NOT_REACHED() MOZ_NOT_REACHED("")
 #define CRASH() MOZ_CRASH()
 #define COMPILE_ASSERT(exp, name) MOZ_STATIC_ASSERT(exp, #name)