diff --git a/diffcore.h b/diffcore.h
index 92788ee4de..c876dac71a 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -46,7 +46,7 @@ struct diff_filespec {
 	unsigned is_stdin : 1;
 	unsigned has_more_entries : 1; /* only appear in combined diff */
 	/* data should be considered "binary"; -1 means "don't know yet" */
-	int is_binary : 2;
+	signed int is_binary : 2;
 	struct userdiff_driver *driver;
 };