--- a/xpcom/string/nsTString.h
+++ b/xpcom/string/nsTString.h
@@ -851,16 +851,17 @@ public:
     Adopt(aStr, aLength);
   }
 
   // copy-constructor required to adopt on copy. Note that this
   // will violate the constness of |aStr| in the operator=()
   // call. |aStr| will be truncated as a side-effect of this
   // constructor.
   nsTAdoptingString_CharT(const self_type& aStr)
+    : nsTXPIDLString_CharT()
   {
     *this = aStr;
   }
 
   // |operator=| does not inherit, so we must define our own
   self_type& operator=(const substring_type& aStr)
   {
     Assign(aStr);