diff --git a/src/util/zbin.c b/src/util/zbin.c
index 0dabaf1e..3b7cf95b 100644
--- a/src/util/zbin.c
+++ b/src/util/zbin.c
@@ -143,7 +143,7 @@ static int alloc_output_file ( size_t max_len, struct output_file *output ) {
 			  max_len );
 		return -1;
 	}
-	memset ( output->buf, 0xff, sizeof ( output->buf ) );
+	memset ( output->buf, 0xff, max_len );
 	return 0;
 }