diff --git a/include/ctype.h b/include/ctype.h
index 6cc2cb8953..67c157e2ee 100644
--- a/include/ctype.h
+++ b/include/ctype.h
@@ -126,7 +126,7 @@
  *
  ************************************************************/
 
-#define isupper(c)   ((c) >= 'a' && (c) <= 'z')
+#define isupper(c)   ((c) >= 'A' && (c) <= 'Z')
 
 /************************************************************
  * Function:  isalpha