diff --git a/testing/web-platform/tests/resources/idlharness.js b/testing/web-platform/tests/resources/idlharness.js
index 5a97b273f2bed..76131e7c9602b 100644
--- a/testing/web-platform/tests/resources/idlharness.js
+++ b/testing/web-platform/tests/resources/idlharness.js
@@ -3543,8 +3543,6 @@ IdlNamespace.prototype.test = function ()
 function idl_test(srcs, deps, idl_setup_func) {
     return promise_test(function (t) {
         var idl_array = new IdlArray();
-        srcs = (srcs instanceof Array) ? srcs : [srcs] || [];
-        deps = (deps instanceof Array) ? deps : [deps] || [];
         var setup_error = null;
         const validationIgnored = [
             "constructor-member",