diff --git a/src/if_py_both.h b/src/if_py_both.h
index f843863..42db510 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -3240,7 +3240,7 @@
     if (self == NULL)
 	return NULL;
 
-    if (isdigit(*name))
+    if (isdigit((unsigned char)*name))
     {
 	if (!translated_function_exists(name, FALSE))
 	{