diff --git a/inputfiles/overridingTypes.jsonc b/inputfiles/overridingTypes.jsonc
index 725751894..12482efc1 100644
--- a/inputfiles/overridingTypes.jsonc
+++ b/inputfiles/overridingTypes.jsonc
@@ -3663,24 +3663,6 @@
                     }
                 }
             },
-            "UnderlyingSource": {
-                "typeParameters": [
-                    {
-                        "name": "R",
-                        "default": "any"
-                    }
-                ],
-                "members": {
-                    "member": {
-                        "start": {
-                            "overrideType": "UnderlyingSourceStartCallback<R>"
-                        },
-                        "pull": {
-                            "overrideType": "UnderlyingSourcePullCallback<R>"
-                        }
-                    }
-                }
-            },
             "UIEventInit": {
                 "members": {
                     "member": {
diff --git a/inputfiles/patches/readable-stream.kdl b/inputfiles/patches/readable-stream.kdl
index 7a8a6f6a0..a3fdbacfd 100644
--- a/inputfiles/patches/readable-stream.kdl
+++ b/inputfiles/patches/readable-stream.kdl
@@ -7,3 +7,9 @@ dictionary ReadableStreamReadValueResult typeParameters=T {
     member done required=#true overrideType="false"
     member value required=#true overrideType="T"
 }
+
+dictionary UnderlyingSource {
+    typeParameters R default = "any"
+    member start overrideType = "UnderlyingSourceStartCallback<R>"
+    member pull overrideType = "UnderlyingSourcePullCallback<R>"
+}
diff --git a/src/build/patches.ts b/src/build/patches.ts
index 7c949ebce..719660e9d 100644
--- a/src/build/patches.ts
+++ b/src/build/patches.ts
@@ -63,14 +63,25 @@ function handleTyped(type: Node): Typed {
   };
 }
 
-function handleTypeParameters(value: Value) {
+function handleTypeParameters(value: Value | Node) {
   if (!value) {
     return {};
   }
+  if (typeof value === "string") {
+    return {
+      typeParameters: [
+        {
+          name: string(value),
+        },
+      ],
+    };
+  }
+  const node = value as Node;
   return {
     typeParameters: [
       {
-        name: string(value),
+        name: string(node.values[0]),
+        ...optionalMember("default", "string", node.properties?.default),
       },
     ],
   };
@@ -318,6 +329,7 @@ function handleMethod(child: Node): DeepPartial<OverridableMethod> {
 function handleDictionary(child: Node): DeepPartial<Dictionary> {
   const name = string(child.values[0]);
   const member: Record<string, Partial<Member>> = {};
+  let typeParameters = {};
 
   for (const c of child.children) {
     switch (c.name) {
@@ -326,6 +338,10 @@ function handleDictionary(child: Node): DeepPartial<Dictionary> {
         member[memberName] = handleMember(c);
         break;
       }
+      case "typeParameters": {
+        typeParameters = handleTypeParameters(c);
+        break;
+      }
       default:
         throw new Error(`Unknown node name: ${c.name}`);
     }
@@ -334,6 +350,7 @@ function handleDictionary(child: Node): DeepPartial<Dictionary> {
   return {
     name,
     members: { member },
+    ...typeParameters,
     ...handleTypeParameters(child.properties?.typeParameters),
     ...optionalMember(
       "legacyNamespace",