From 7da613aa94231335c13762ae93105b421c00e06f Mon Sep 17 00:00:00 2001
From: gopherclass <gopherclass@gmail.com>
Date: Mon, 9 Mar 2020 19:37:51 +0900
Subject: [PATCH] Fix panic while writing invalid ini

---
 viper.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/viper.go b/viper.go
index 7b12b36..823339c 100644
--- a/viper.go
+++ b/viper.go
@@ -1616,6 +1616,10 @@ func (v *Viper) marshalWriter(f afero.File, configType string) error {
 		for i := 0; i < len(keys); i++ {
 			key := keys[i]
 			lastSep := strings.LastIndex(key, ".")
+			if lastSep < 0 {
+				err := fmt.Errorf("missing key for %s", key)
+				return ConfigMarshalError{err}
+			}
 			sectionName := key[:(lastSep)]
 			keyName := key[(lastSep + 1):]
 			if sectionName == "default" {