diff --git a/docsgpt/deploy/commands.py b/docsgpt/deploy/commands.py
index 19babb5a..db114639 100644
--- a/docsgpt/deploy/commands.py
+++ b/docsgpt/deploy/commands.py
@@ -215,6 +215,13 @@ def _redis_urls(base: str) -> dict[str, str]:
     except ValueError as exc:
         # urlsplit raises on things like redis://[::1 ; that is a typo, not a crash.
         raise DeployError(f"the Redis URL {base!r} could not be read: {exc}") from exc
+    try:
+        _ = parts.port  # a non-numeric or out-of-range port raises here, not when the URL is split
+    except ValueError as exc:
+        raise DeployError(
+            f"the Redis URL {base!r} has an unusable port: {exc}. Pass a URL like "
+            "redis://host:6379 or redis://host:6379/5."
+        ) from exc
     if parts.scheme not in ("redis", "rediss"):
         raise DeployError(
             f"the Redis URL {base!r} should start with redis:// or rediss://, with any options as "
diff --git a/tests/deploy/test_native.py b/tests/deploy/test_native.py
index 33a50568..a9335068 100644
--- a/tests/deploy/test_native.py
+++ b/tests/deploy/test_native.py
@@ -231,6 +231,15 @@ class TestNativeUp:
         with pytest.raises(DeployError, match="too long to read"):
             _run(argv, _native_context())
 
+    @pytest.mark.parametrize("url", ["redis://localhost:notaport/0", "redis://localhost:65536/0"])
+    def test_a_redis_url_with_an_unusable_port_is_refused(self, tmp_path, url):
+        """urlsplit accepts it and rebuilds it verbatim; only .port notices, and the worker dies later."""
+        argv = ["up", "--native", "--dir", str(tmp_path), "--yes",
+                "--postgres-uri", "postgresql://localhost/d", "--redis-url", url]
+        with pytest.raises(DeployError, match="unusable port"):
+            _run(argv, _native_context())
+        assert not (tmp_path / ".env").exists(), "it refuses before writing settings the worker would read"
+
     @pytest.mark.parametrize("url", ["localhost:6379", "redis+socket:///var/run/redis.sock",
                                      "redis://localhost:6379/queue"])
     def test_a_redis_url_that_cannot_be_numbered_is_refused(self, tmp_path, url):