diff --git a/IMPROVEMENTS.md b/IMPROVEMENTS.md
index a0766e8..1b28d3f 100644
--- a/IMPROVEMENTS.md
+++ b/IMPROVEMENTS.md
@@ -199,7 +199,9 @@ Acceptance criteria:
   since it can't be invoked from an already non-root process anyway.
   Verified against the real image (not just mocked unit tests): a running
   container's PID 1 shows `Uid: 1000 1000 1000 1000` in `/proc/1/status`.
-- [ ] Reject UID or GID zero unless a documented use case requires it.
+- [x] Reject UID or GID zero unless a documented use case requires it.
+  `main()` now exits with code 2 and a clear message if `--uid`/`SEAFILE_UID`
+  or `--gid`/`SEAFILE_GID` resolve to 0, before `setup_uid()` ever runs.
 - [ ] Add a health check for the daemon and RPC endpoint.
 - [ ] Handle temporary RPC failures and daemon restarts without corrupting
   state.
diff --git a/start.py b/start.py
index d630758..45dc498 100755
--- a/start.py
+++ b/start.py
@@ -131,6 +131,12 @@ def main():
         parser.error("username is not specified")
     if not args.libraries:
         parser.error("library is not specified")
+    if args.uid == 0:
+        parser.error("uid must not be 0; running the daemon as root defeats "
+                      "the point of --uid/SEAFILE_UID")
+    if args.gid == 0:
+        parser.error("gid must not be 0 (the root group); running the daemon "
+                      "with it defeats the point of --gid/SEAFILE_GID")
 
     try:
         password = resolve_secret("PASSWORD", args.password, args.password_file)
diff --git a/tests/smoke.sh b/tests/smoke.sh
index 7def2f6..906c95d 100755
--- a/tests/smoke.sh
+++ b/tests/smoke.sh
@@ -139,6 +139,22 @@ else
     fail "negative limit not reported: $(echo "$out" | tail -1)"
 fi
 
+out=$(docker run --rm $PLATFORM_ARG -e SERVER_HOST=h -e USERNAME=u -e PASSWORD=p \
+    -e LIBRARY_ID=x -e SEAFILE_UID=0 "$IMAGE" 2>&1; echo "rc=$?")
+if echo "$out" | grep -q "uid must not be 0" && [ "$(echo "$out" | tail -1)" = "rc=2" ]; then
+    pass "SEAFILE_UID=0 is rejected"
+else
+    fail "SEAFILE_UID=0 not rejected: $out"
+fi
+
+out=$(docker run --rm $PLATFORM_ARG -e SERVER_HOST=h -e USERNAME=u -e PASSWORD=p \
+    -e LIBRARY_ID=x -e SEAFILE_GID=0 "$IMAGE" 2>&1; echo "rc=$?")
+if echo "$out" | grep -q "gid must not be 0" && [ "$(echo "$out" | tail -1)" = "rc=2" ]; then
+    pass "SEAFILE_GID=0 is rejected"
+else
+    fail "SEAFILE_GID=0 not rejected: $out"
+fi
+
 echo "== a mounted secret is read and stays out of the environment"
 printf 's3cret-from-file\n' > "$WORK/pw"
 out=$(docker run --rm $PLATFORM_ARG -v "$WORK/pw:/run/secrets/pw:ro" \