diff --git a/frontend/src/core/uploads/api.ts b/frontend/src/core/uploads/api.ts
index 501cfe8c04..769f314979 100644
--- a/frontend/src/core/uploads/api.ts
+++ b/frontend/src/core/uploads/api.ts
@@ -147,7 +147,10 @@ export async function uploadFiles(
 
   const result = (await response.json()) as UploadResponse;
   if (result.success === false) {
-    const message = result.message.trim();
+    // A gateway or proxy may omit `message`; treat it like an empty one
+    // instead of throwing a TypeError while handling the failure.
+    const message =
+      typeof result.message === "string" ? result.message.trim() : "";
     throw new Error(
       message ? `Upload incomplete: ${message}` : "Upload failed",
     );
diff --git a/frontend/tests/unit/core/uploads/api.test.ts b/frontend/tests/unit/core/uploads/api.test.ts
index 105d307e7d..7b7fe19d6f 100644
--- a/frontend/tests/unit/core/uploads/api.test.ts
+++ b/frontend/tests/unit/core/uploads/api.test.ts
@@ -94,6 +94,18 @@ describe("uploads api", () => {
     expect(error.message).toBe("Upload failed");
   });
 
+  test("uses the upload fallback when an unsuccessful response omits the message", async () => {
+    const error = await uploadError(
+      jsonResponse(200, {
+        success: false,
+        files: [],
+        skipped_files: ["report.pdf"],
+      }),
+    );
+
+    expect(error.message).toBe("Upload failed");
+  });
+
   test("preserves string error details", async () => {
     const error = await uploadError(
       jsonResponse(413, { detail: "File exceeds the upload limit" }),