diff --git a/packages/client/src/object/convertWireToOsdkObjects.test.ts b/packages/client/src/object/convertWireToOsdkObjects.test.ts
index 07ee728f4..d512c3f80 100644
--- a/packages/client/src/object/convertWireToOsdkObjects.test.ts
+++ b/packages/client/src/object/convertWireToOsdkObjects.test.ts
@@ -14,12 +14,14 @@
  * limitations under the License.
  */
 
+import { createClientContext } from "@osdk/shared.net";
 import { apiServer } from "@osdk/shared.test";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import type { Client } from "../Client.js";
 import { createClient } from "../createClient.js";
 import { Ontology as MockOntology } from "../generatedNoCheck/index.js";
 import { Attachment } from "./Attachment.js";
+import { convertWireToOsdkObjects } from "./convertWireToOsdkObjects.js";
 
 describe("convertWireToOsdkObjects", () => {
   let client: Client<typeof MockOntology>;
@@ -65,4 +67,23 @@ describe("convertWireToOsdkObjects", () => {
     expect(emptyAttachment).toBeUndefined();
     expect(emptyAttachmentArray).toBeUndefined();
   });
+
+  it("works even with unknown apiNames", () => {
+    const clientCtx = createClientContext(
+      MockOntology,
+      "https://stack.palantir.com",
+      () => "myAccessToken",
+      "userAgent",
+    );
+
+    const object = {
+      __apiName: "unknown",
+      __primaryKey: 0,
+    } as const;
+    const prototypeBefore = Object.getPrototypeOf(object);
+    convertWireToOsdkObjects(clientCtx, [object]);
+    const prototypeAfter = Object.getPrototypeOf(object);
+
+    expect(prototypeBefore).not.toBe(prototypeAfter);
+  });
 });
diff --git a/packages/client/src/object/convertWireToOsdkObjects.ts b/packages/client/src/object/convertWireToOsdkObjects.ts
index 3b0e7ff10..5f7ad8b5c 100644
--- a/packages/client/src/object/convertWireToOsdkObjects.ts
+++ b/packages/client/src/object/convertWireToOsdkObjects.ts
@@ -39,6 +39,10 @@ function createPrototype<
   const objDef = ontology.objects[type];
   const proto = {};
 
+  if (!objDef) {
+    return proto;
+  }
+
   Object.defineProperty(proto, "$link", {
     get: function() {
       const client = this[OriginClient] as ClientContext<O>;
@@ -116,10 +120,15 @@ function createConverter<
   ontology: O,
   type: T,
 ) {
+  const objDef = ontology.objects[type];
+  if (!objDef) {
+    return false as const;
+  }
+
   const steps: Array<(o: Record<string, any>) => void> = [];
 
   for (
-    const [key, value] of Object.entries(ontology.objects[type].properties)
+    const [key, value] of Object.entries(objDef.properties)
   ) {
     // attachments need a wrapper to provide functionality and to identify them at serialization time
     if (value.type === "attachment") {