diff --git a/sleep.c b/sleep.c
index 8ed32f7..5c2d2f4 100644
--- a/sleep.c
+++ b/sleep.c
@@ -23,7 +23,7 @@ main(int argc, char *argv[])
 	if (argc < 1)
 		usage();
 
-	seconds = estrtol(argv[0], 0);
+	seconds = estrtoul(argv[0], 0);
 	while ((seconds = sleep(seconds)) > 0)
 		;
 	return 0;