diff --git a/js/xpconnect/src/XPCConvert.cpp b/js/xpconnect/src/XPCConvert.cpp
index 9f5de16d92a..ae0b37d0da6 100644
--- a/js/xpconnect/src/XPCConvert.cpp
+++ b/js/xpconnect/src/XPCConvert.cpp
@@ -516,6 +516,14 @@ XPCConvert::JSData2Native(void* d, HandleValue s,
 
         if (!str) {
             ws->AssignLiteral(MOZ_UTF16("undefined"));
+        } else if (XPCStringConvert::IsDOMString(str)) {
+            // The characters represent an existing nsStringBuffer that
+            // was shared by XPCStringConvert::ReadableToJSVal.
+            nsStringBuffer::FromData((void *)chars)->ToString(length, *ws);
+        } else if (XPCStringConvert::IsLiteral(str)) {
+            // The characters represent a literal char16_t string constant
+            // compiled into libxul, such as the string "undefined" above.
+            ws->AssignLiteral(chars, length);
         } else if (useAllocator && STRING_TO_JSVAL(str) == s) {
             // The JS string will exist over the function call.
             // We don't need to copy the characters in this case.
diff --git a/js/xpconnect/src/xpcpublic.h b/js/xpconnect/src/xpcpublic.h
index 9a3165a5145..63ba9cd6de2 100644
--- a/js/xpconnect/src/xpcpublic.h
+++ b/js/xpconnect/src/xpcpublic.h
@@ -234,6 +234,18 @@ public:
     static void FreeZoneCache(JS::Zone *zone);
     static void ClearZoneCache(JS::Zone *zone);
 
+    static MOZ_ALWAYS_INLINE bool IsLiteral(JSString *str)
+    {
+        return JS_IsExternalString(str) &&
+               JS_GetExternalStringFinalizer(str) == &sLiteralFinalizer;
+    }
+
+    static MOZ_ALWAYS_INLINE bool IsDOMString(JSString *str)
+    {
+        return JS_IsExternalString(str) &&
+               JS_GetExternalStringFinalizer(str) == &sDOMStringFinalizer;
+    }
+
 private:
     static const JSStringFinalizer sLiteralFinalizer, sDOMStringFinalizer;
 
diff --git a/xpcom/string/public/nsTSubstring.h b/xpcom/string/public/nsTSubstring.h
index 8eef2c0ddb9..36b609153a6 100644
--- a/xpcom/string/public/nsTSubstring.h
+++ b/xpcom/string/public/nsTSubstring.h
@@ -834,13 +834,16 @@ class nsTSubstring_CharT
           mFlags = dataFlags | (mFlags & 0xFFFF0000);
         }
 
-      void NS_FASTCALL AssignLiteral( const char_type* data, size_type length );
       void NS_FASTCALL ReplaceLiteral( index_type cutStart, size_type cutLength, const char_type* data, size_type length );
 
       static int AppendFunc( void* arg, const char* s, uint32_t len);
 
     public:
 
+      // NOTE: this method is declared public _only_ for convenience for
+      // callers who don't have access to the original nsLiteralString_CharT.
+      void NS_FASTCALL AssignLiteral( const char_type* data, size_type length );
+
       // mFlags is a bitwise combination of the following flags.  the meaning
       // and interpretation of these flags is an implementation detail.
       //