diff --git a/ext/bigdecimal/missing.h b/ext/bigdecimal/missing.h
index 307147c0fd..325554b5f5 100644
--- a/ext/bigdecimal/missing.h
+++ b/ext/bigdecimal/missing.h
@@ -35,10 +35,10 @@ extern "C" {
 #endif /* RB_UNUSED_VAR */
 
 #if defined(_MSC_VER) && _MSC_VER >= 1310
-# define HAVE___ASSUME
+# define HAVE___ASSUME 1
 
 #elif defined(__INTEL_COMPILER) && __INTEL_COMPILER >= 1300
-# define HAVE___ASSUME
+# define HAVE___ASSUME 1
 #endif
 
 #ifndef UNREACHABLE