diff --git a/src/filepath.c b/src/filepath.c
index 1ea0623..cbf2da4 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -2953,7 +2953,7 @@
 
 #if defined(MSWIN)
     // may skip "c:"
-    if (isalpha(path[0]) && path[1] == ':')
+    if (SAFE_isalpha(path[0]) && path[1] == ':')
 	retval = path + 2;
     else
 	retval = path;