diff --git a/js/src/xpconnect/src/xpcwrappednative.cpp b/js/src/xpconnect/src/xpcwrappednative.cpp
index 6d99a5d6cff3..14754702eae8 100644
--- a/js/src/xpconnect/src/xpcwrappednative.cpp
+++ b/js/src/xpconnect/src/xpcwrappednative.cpp
@@ -2848,12 +2848,9 @@ CallMethodHelper::ConvertIndependentParam(uint8 i)
     // indirectly, regardless of in/out-ness.
     if(type_tag == nsXPTType::T_JSVAL)
     {
-        // Assign the value
-        JS_STATIC_ASSERT(sizeof(jsval) <= sizeof(dp->val));
-        jsval *rootp = (jsval *)&dp->val;
-        dp->ptr = rootp;
-        *rootp = JSVAL_VOID;
-        if(!JS_AddValueRoot(mCallContext, rootp))
+        // Root the value.
+        dp->val.j = JSVAL_VOID;
+        if (!JS_AddValueRoot(mCallContext, &dp->val.j))
             return JS_FALSE;
         dp->SetValIsJSRoot();
     }
diff --git a/xpcom/reflect/xptcall/public/xptcall.h b/xpcom/reflect/xptcall/public/xptcall.h
index f58b5ec93540..1ac59195823e 100644
--- a/xpcom/reflect/xptcall/public/xptcall.h
+++ b/xpcom/reflect/xptcall/public/xptcall.h
@@ -52,6 +52,7 @@
 #include "nsISupports.h"
 #include "xpt_struct.h"
 #include "xptinfo.h"
+#include "jsapi.h"
 
 struct nsXPTCMiniVariant
 {
@@ -73,6 +74,11 @@ struct nsXPTCMiniVariant
         char      c;
         PRUnichar wc;
         void*     p;
+
+        // Types below here are unknown to the assembly implementations, and
+        // therefore _must_ be passed with indirect semantics. We put them in
+        // the union here for type safety, so that we can avoid void* tricks.
+        jsval j;
     } val;
 };