diff --git a/src/core/worker.js b/src/core/worker.js
index eb819a2df..5ec40561d 100644
--- a/src/core/worker.js
+++ b/src/core/worker.js
@@ -454,8 +454,8 @@ class WorkerMessageHandler {
       });
     });
 
-    handler.on("GetPageIndex", function wphSetupGetPageIndex({ ref }) {
-      const pageRef = Ref.get(ref.num, ref.gen);
+    handler.on("GetPageIndex", function wphSetupGetPageIndex(data) {
+      const pageRef = Ref.get(data.num, data.gen);
       return pdfManager.ensureCatalog("getPageIndex", [pageRef]);
     });
 
diff --git a/src/display/api.js b/src/display/api.js
index 0fd2f71e5..241db612e 100644
--- a/src/display/api.js
+++ b/src/display/api.js
@@ -2879,8 +2879,19 @@ class WorkerTransport {
   }
 
   getPageIndex(ref) {
+    if (
+      typeof ref !== "object" ||
+      ref === null ||
+      !Number.isInteger(ref.num) ||
+      ref.num < 0 ||
+      !Number.isInteger(ref.gen) ||
+      ref.gen < 0
+    ) {
+      return Promise.reject(new Error("Invalid pageIndex request."));
+    }
     return this.messageHandler.sendWithPromise("GetPageIndex", {
-      ref,
+      num: ref.num,
+      gen: ref.gen,
     });
   }
 
diff --git a/test/unit/api_spec.js b/test/unit/api_spec.js
index 54c1ef885..273aa4be0 100644
--- a/test/unit/api_spec.js
+++ b/test/unit/api_spec.js
@@ -890,18 +890,35 @@ describe("api", function () {
     });
 
     it("gets invalid page index", async function () {
-      const ref = { num: 3, gen: 0 }; // Reference to a font dictionary.
+      const pageRefs = [
+        /* fontRef = */ { num: 3, gen: 0 },
+        /* invalidRef = */ { num: -1, gen: 0 },
+        /* nonRef = */ "qwerty",
+        /* nullRef = */ null,
+      ];
 
-      try {
-        await pdfDocument.getPageIndex(ref);
+      const expectedErrors = [
+        {
+          exception: UnknownErrorException,
+          message: "The reference does not point to a /Page dictionary.",
+        },
+        { exception: Error, message: "Invalid pageIndex request." },
+        { exception: Error, message: "Invalid pageIndex request." },
+        { exception: Error, message: "Invalid pageIndex request." },
+      ];
 
-        // Shouldn't get here.
-        expect(false).toEqual(true);
-      } catch (reason) {
-        expect(reason instanceof UnknownErrorException).toEqual(true);
-        expect(reason.message).toEqual(
-          "The reference does not point to a /Page dictionary."
-        );
+      for (let i = 0, ii = pageRefs.length; i < ii; i++) {
+        try {
+          await pdfDocument.getPageIndex(pageRefs[i]);
+
+          // Shouldn't get here.
+          expect(false).toEqual(true);
+        } catch (reason) {
+          const { exception, message } = expectedErrors[i];
+
+          expect(reason instanceof exception).toEqual(true);
+          expect(reason.message).toEqual(message);
+        }
       }
     });