diff --git a/js/src/xpconnect/src/xpccomponents.cpp b/js/src/xpconnect/src/xpccomponents.cpp
index f4f3c3ada67e..eb80b571ad50 100644
--- a/js/src/xpconnect/src/xpccomponents.cpp
+++ b/js/src/xpconnect/src/xpccomponents.cpp
@@ -2194,7 +2194,8 @@ nsXPCComponents_Util::EvalInSandbox(const nsAString &source,
     JS_SetErrorReporter(cx, borrowedReporter);
 
     if(!JS_EvaluateUCScriptForPrincipals(sandcx, sandbox, jsPrincipals,
-                                         PromiseFlatString(source).get(),
+                                         NS_REINTERPRET_CAST(const jschar *,
+                                             PromiseFlatString(source).get()),
                                          source.Length(),
                                          PromiseFlatUTF8String(codebase).get(),
                                          1, rval)) {