diff --git a/config-merge.js b/config-merge.js
index ae2a84d..fc39a10 100644
--- a/config-merge.js
+++ b/config-merge.js
@@ -238,7 +238,10 @@ if (format == "json") {
         }
     })
 } else {
-    serialized = YAML.stringify(obj, options={simpleKeys: true})
+    serialized = YAML.stringify(obj, {
+        defaultStringType: 'QUOTE_DOUBLE',
+        simpleKeys: true
+    })
 }
 
 if (format == "json" || format == "json5" || format == "toml") {