diff --git a/viper.go b/viper.go
index f61f4ed..ad8b658 100644
--- a/viper.go
+++ b/viper.go
@@ -1649,7 +1649,7 @@ func (v *Viper) marshalWriter(f afero.File, configType string) error {
 			if sectionName == "default" {
 				sectionName = ""
 			}
-			cfg.Section(sectionName).Key(keyName).SetValue(v.Get(key).(string))
+			cfg.Section(sectionName).Key(keyName).SetValue(cast.ToString(v.Get(key)))
 		}
 		cfg.WriteTo(f)
 	}