diff --git a/tests/checkasm/Makefile b/tests/checkasm/Makefile
index 483ad136f0..4c0ffab884 100644
--- a/tests/checkasm/Makefile
+++ b/tests/checkasm/Makefile
@@ -18,7 +18,7 @@ $(CHECKASMOBJS): | $(CHECKASMDIRS)
 OBJDIRS += $(CHECKASMDIRS)
 
 # We rely on function pointers intentionally declared without specified argument types.
-tests/checkasm/%.o: CFLAGS := $(CFLAGS:-Wstrict-prototypes=-Wno-strict-prototypes)
+tests/checkasm/%.o: CFLAGS := $(CFLAGS:-Wstrict-prototypes=-Wno-strict-prototypes) -Umain
 
 CHECKASM := tests/checkasm/checkasm$(EXESUF)