diff --git a/src/dbus_fast/auth.py b/src/dbus_fast/auth.py
index 09fa6f6b..5ce353e5 100644
--- a/src/dbus_fast/auth.py
+++ b/src/dbus_fast/auth.py
@@ -1,6 +1,6 @@
 import enum
 import os
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 
 from .errors import AuthError
 
@@ -60,13 +60,17 @@ class AuthExternal(Authenticator):
     :sealso: https://dbus.freedesktop.org/doc/dbus-specification.html#auth-protocol
     """
 
-    def __init__(self) -> None:
+    def __init__(self, uid: int = None) -> None:
         self.negotiate_unix_fd: bool = False
         self.negotiating_fds: bool = False
+        self.uid: Optional[int] = uid
 
     def _authentication_start(self, negotiate_unix_fd: bool = False) -> str:
         self.negotiate_unix_fd = negotiate_unix_fd
-        hex_uid = str(os.getuid()).encode().hex()
+        uid = self.uid
+        if uid is None:
+            uid = os.getuid()
+        hex_uid = str(uid).encode().hex()
         return f"AUTH EXTERNAL {hex_uid}"
 
     def _receive_line(self, line: str) -> str:
diff --git a/tests/test_auth.py b/tests/test_auth.py
new file mode 100644
index 00000000..4fec0002
--- /dev/null
+++ b/tests/test_auth.py
@@ -0,0 +1,11 @@
+"""This tests setting a hardcoded UID in AuthExternal"""
+import os
+
+import pytest
+
+from dbus_fast.auth import AuthExternal
+
+
+def test_uid_is_set():
+    auth = AuthExternal(uid=999)
+    assert auth._authentication_start() == "AUTH EXTERNAL 393939"