diff --git a/src/components/atoms/OakIcon/OakIcon.test.tsx b/src/components/atoms/OakIcon/OakIcon.test.tsx
index 547009a5..c8fc7112 100644
--- a/src/components/atoms/OakIcon/OakIcon.test.tsx
+++ b/src/components/atoms/OakIcon/OakIcon.test.tsx
@@ -75,6 +75,17 @@ describe("generateOakIconURL", () => {
     expect(generateOakIconURL("home")).toBe(
       "https://res.cloudinary.com/mock-cloudinary-cloud/image/upload/v1699887218/icons/gvqxjxcw07ei2kkmwnes.svg",
     );
-    expect(generateOakIconURL("banana-sandwich")).toBe(undefined);
+  });
+
+  it("is returns url for question mark when the string is not a valid icon name", () => {
+    expect(generateOakIconURL("banana-sandwich")).toBe(
+      "https://res.cloudinary.com/mock-cloudinary-cloud/image/upload/v1706872277/icons/question-mark.svg",
+    );
+  });
+
+  it("is returns url for books when the string is not a valid subject icon name", () => {
+    expect(generateOakIconURL("subject-potions")).toBe(
+      "https://res.cloudinary.com/mock-cloudinary-cloud/image/upload/v1699953657/icons/hz4l3iq6i68kazvkvorq.svg",
+    );
   });
 });