diff --git a/server/app.py b/server/app.py
index 4853b24..bf39010 100644
--- a/server/app.py
+++ b/server/app.py
@@ -2,6 +2,7 @@
 import os
 
 from fastapi import Depends, FastAPI, Header, HTTPException, Query
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 
 from . import actions, jobs, mf
@@ -23,6 +24,11 @@ async def _http_exc(_req, exc: HTTPException):
     return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})
 
 
+@app.exception_handler(RequestValidationError)
+async def _validation_exc(_req, exc: RequestValidationError):
+    return JSONResponse(status_code=422, content={"message": "Invalid or missing request parameters."})
+
+
 @app.get("/health")
 def health():
     return {"status": "ok"}
diff --git a/tests/test_api.py b/tests/test_api.py
index 0df91c4..8c63d15 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -45,6 +45,7 @@ def test_fetch_no_hits_returns_404(client, auth, monkeypatch):
 def test_fetch_missing_q_returns_422(client, auth):
     r = client.post("/fetch", headers=auth)
     assert r.status_code == 422
+    assert "message" in r.json()
 
 
 def test_job_lifecycle_done(client, auth, monkeypatch):