diff --git a/clip_osx.mm b/clip_osx.mm
index a862145..3591ddd 100644
--- a/clip_osx.mm
+++ b/clip_osx.mm
@@ -230,7 +230,7 @@ bool get_image_from_clipboard(NSPasteboard* pasteboard,
 
     if (f == text_format()) {
       NSString* string = [pasteboard stringForType:NSPasteboardTypeString];
-      int reqsize = [string lengthOfBytesUsingEncoding:NSUTF8StringEncoding]+1;
+      size_t reqsize = [string lengthOfBytesUsingEncoding:NSUTF8StringEncoding]+1;
 
       assert(reqsize <= len);
       if (reqsize > len) {