diff --git a/modules/log/flags.go b/modules/log/flags.go
index 992fc62ddb..95950548ec 100644
--- a/modules/log/flags.go
+++ b/modules/log/flags.go
@@ -60,5 +60,8 @@ func FlagsFromString(from string) int {
 			flags |= f
 		}
 	}
+	if flags == 0 {
+		return -1
+	}
 	return flags
 }