diff --git a/lib/web/fetch/webidl.js b/lib/web/fetch/webidl.js
index 9db13d07df7..d3cc4791550 100644
--- a/lib/web/fetch/webidl.js
+++ b/lib/web/fetch/webidl.js
@@ -354,12 +354,9 @@ webidl.interfaceConverter = function (i) {
 
 webidl.dictionaryConverter = function (converters) {
   return (dictionary, prefix, argument) => {
-    const type = webidl.util.Type(dictionary)
     const dict = {}
 
-    if (type === 'Null' || type === 'Undefined') {
-      return dict
-    } else if (type !== 'Object') {
+    if (dictionary != null && webidl.util.Type(dictionary) !== 'Object') {
       throw webidl.errors.exception({
         header: prefix,
         message: `Expected ${dictionary} to be one of: Null, Undefined, Object.`
@@ -370,7 +367,7 @@ webidl.dictionaryConverter = function (converters) {
       const { key, defaultValue, required, converter } = options
 
       if (required === true) {
-        if (!Object.hasOwn(dictionary, key)) {
+        if (dictionary == null || !Object.hasOwn(dictionary, key)) {
           throw webidl.errors.exception({
             header: prefix,
             message: `Missing required key "${key}".`
@@ -378,13 +375,13 @@ webidl.dictionaryConverter = function (converters) {
         }
       }
 
-      let value = dictionary[key]
-      const hasDefault = Object.hasOwn(options, 'defaultValue')
+      let value = dictionary?.[key]
+      const hasDefault = defaultValue !== undefined
 
       // Only use defaultValue if value is undefined and
       // a defaultValue options was provided.
-      if (hasDefault && value !== null) {
-        value ??= defaultValue()
+      if (hasDefault && value === undefined) {
+        value = defaultValue()
       }
 
       // A key can be optional and have no default value.
diff --git a/test/webidl/converters.js b/test/webidl/converters.js
index e88a54a48b6..ccc376a7ab9 100644
--- a/test/webidl/converters.js
+++ b/test/webidl/converters.js
@@ -76,6 +76,51 @@ describe('webidl.dictionaryConverter', () => {
       converter({ Key: 'this key was required!' }, 'converter', 'converter')
     })
   })
+
+  test('null and undefined still populates defaultValue(s)', () => {
+    const dict = webidl.dictionaryConverter([
+      {
+        key: 'key',
+        converter: webidl.converters.any,
+        defaultValue: () => 3
+      }
+    ])
+
+    assert.deepStrictEqual(dict(null), { key: 3 })
+    assert.deepStrictEqual(dict(undefined), { key: 3 })
+  })
+
+  test('null and undefined throw a webidl TypeError with a required key', () => {
+    const dict = webidl.dictionaryConverter([
+      {
+        key: 'key',
+        converter: webidl.converters.any,
+        required: true
+      }
+    ])
+
+    assert.throws(() => dict(null, 'prefix'), new TypeError('prefix: Missing required key "key".'))
+    assert.throws(() => dict(undefined, 'prefix'), new TypeError('prefix: Missing required key "key".'))
+  })
+
+  test('Object type works for functions and regex (etc.)', () => {
+    const dict = webidl.dictionaryConverter([
+      {
+        key: 'key',
+        converter: webidl.converters.any,
+        required: true
+      }
+    ])
+
+    function obj () {}
+    obj.key = 1
+
+    const obj2 = / /
+    obj2.key = 1
+
+    assert.deepStrictEqual(dict(obj), { key: 1 })
+    assert.deepStrictEqual(dict(obj2), { key: 1 })
+  })
 })
 
 test('ArrayBuffer', () => {