diff --git a/xpcom/glue/nsStringAPI.h b/xpcom/glue/nsStringAPI.h
index fa473c8d704a..19279481a7f2 100644
--- a/xpcom/glue/nsStringAPI.h
+++ b/xpcom/glue/nsStringAPI.h
@@ -1225,6 +1225,14 @@ public:
                       NS_CSTRING_ENCODING_UTF8, *this);
   }
 
+#ifdef MOZ_USE_CHAR16_WRAPPER
+  explicit NS_ConvertUTF16toUTF8(char16ptr_t aString,
+                                 uint32_t aLength = UINT32_MAX)
+    : NS_ConvertUTF16toUTF8(static_cast<const char16_t*>(aString))
+  {
+  }
+#endif
+
 private:
   self_type& operator=(const self_type& aString) = delete;
 };