diff --git a/lib/math.c b/lib/math.c
index 2e53b09..19c8f08 100644
--- a/lib/math.c
+++ b/lib/math.c
@@ -306,7 +306,7 @@
 		/* treat undefined var as 0 */
 		t->val = 0;
 	}
-	return 0;
+	return NULL;
 }
 
 /* "Applying" a token means performing it on the top elements on the integer