--- a/js/xpconnect/src/XPCWrappedNativeScope.cpp
+++ b/js/xpconnect/src/XPCWrappedNativeScope.cpp
@@ -287,21 +287,20 @@ XPCWrappedNativeScope::EnsureContentXBLS
     SandboxOptions options;
     options.wantXrays = false;
     options.wantComponents = true;
     options.proto = global;
     options.sameZoneAs = global;
 
     // Use an nsExpandedPrincipal to create asymmetric security.
     nsIPrincipal* principal = GetPrincipal();
-    nsCOMPtr<nsIExpandedPrincipal> ep;
-    MOZ_ASSERT(!(ep = do_QueryInterface(principal)));
+    MOZ_ASSERT(!nsContentUtils::IsExpandedPrincipal(principal));
     nsTArray< nsCOMPtr<nsIPrincipal> > principalAsArray(1);
     principalAsArray.AppendElement(principal);
-    ep = new nsExpandedPrincipal(principalAsArray);
+    nsCOMPtr<nsIExpandedPrincipal> ep = new nsExpandedPrincipal(principalAsArray);
 
     // Create the sandbox.
     RootedValue v(cx);
     nsresult rv = CreateSandboxObject(cx, &v, ep, options);
     NS_ENSURE_SUCCESS(rv, nullptr);
     mContentXBLScope = &v.toObject();
 
     // Tag it.