diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index 14a26268efd..226587b5283 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -1184,22 +1184,23 @@ class CGArgumentConverter(CGThing):
                 template, variadicConversion, declType, "slot")]
 
             arg = "arg%d" % index
-            vec = "Vec::with_capacity((%(argc)s - %(index)s) as usize)" % {'argc': argc, 'index': index}
 
             if argument.type.isGeckoInterface():
-                code = "let mut %s = RootedVec::new();\n*%s = %s;\n" % (arg, arg, vec)
+                vec = "RootedVec::new()"
                 innerConverter.append(CGGeneric("%s.push(JS::from_ref(&*slot));" % arg))
             else:
-                code = "let mut %s = %s;\n" % (arg, vec)
+                vec = "vec![]"
                 innerConverter.append(CGGeneric("%s.push(slot);" % arg))
-            inner = CGIndenter(CGList(innerConverter, "\n"), 4).define()
+            inner = CGIndenter(CGList(innerConverter, "\n"), 8).define()
 
-            code += """\
-for variadicArg in %(index)s..%(argc)s {
+            self.converter = CGGeneric("""\
+let mut %(arg)s = %(vec)s;
+if %(argc)s > %(index)s {
+    %(arg)s.reserve(%(argc)s as usize - %(index)s);
+    for variadicArg in %(index)s..%(argc)s {
 %(inner)s
-}""" % {'argc': argc, 'index': index, 'inner': inner}
-
-            self.converter = CGGeneric(code)
+    }
+}""" % {'arg': arg, 'argc': argc, 'index': index, 'inner': inner, 'vec': vec})
 
     def define(self):
         return self.converter.define()
diff --git a/components/script/dom/testbinding.rs b/components/script/dom/testbinding.rs
index df098b98aa5..bd9515d55bf 100644
--- a/components/script/dom/testbinding.rs
+++ b/components/script/dom/testbinding.rs
@@ -362,6 +362,7 @@ impl TestBindingMethods for TestBinding {
     // fn PassOptionalNullableEnumWithNonNullDefault(self, _: Option<TestEnum>) {}
 
     fn PassVariadicBoolean(&self, _: Vec<bool>) {}
+    fn PassVariadicBooleanAndDefault(&self, _: bool, _: Vec<bool>) {}
     fn PassVariadicByte(&self, _: Vec<i8>) {}
     fn PassVariadicOctet(&self, _: Vec<u8>) {}
     fn PassVariadicShort(&self, _: Vec<i16>) {}
diff --git a/components/script/dom/webidls/TestBinding.webidl b/components/script/dom/webidls/TestBinding.webidl
index 803944b75fa..419f5e6ad19 100644
--- a/components/script/dom/webidls/TestBinding.webidl
+++ b/components/script/dom/webidls/TestBinding.webidl
@@ -325,6 +325,7 @@ interface TestBinding {
   // void passOptionalNullableUnion2WithNonNullDefault(optional (Event or DOMString)? data = "foo");
 
   void passVariadicBoolean(boolean... args);
+  void passVariadicBooleanAndDefault(optional boolean arg = true, boolean... args);
   void passVariadicByte(byte... args);
   void passVariadicOctet(octet... args);
   void passVariadicShort(short... args);
diff --git a/tests/wpt/mozilla/tests/mozilla/variadic-interface.html b/tests/wpt/mozilla/tests/mozilla/variadic-interface.html
index bf928f8fe09..5ab0557c5e0 100644
--- a/tests/wpt/mozilla/tests/mozilla/variadic-interface.html
+++ b/tests/wpt/mozilla/tests/mozilla/variadic-interface.html
@@ -5,7 +5,12 @@
 <script src=/resources/testharnessreport.js></script>
 <script>
 test(function() {
-	var t = new TestBinding;
-	t.passVariadicInterface(new Blob, new Blob);
+    var t = new TestBinding;
+    t.passVariadicInterface(new Blob, new Blob);
 }, "Variadic interface arguments work.");
+
+test(function() {
+    var t = new TestBinding;
+    t.passVariadicBooleanAndDefault();
+}, "Default values and variadic arguments work together.");
 </script>