diff --git a/tests/mock_pchk.py b/tests/mock_pchk.py
index 1734d5b..70f0e26 100644
--- a/tests/mock_pchk.py
+++ b/tests/mock_pchk.py
@@ -129,6 +129,7 @@ async def authentication(self) -> bool:
 
         if self.license_error:
             self.writer.write(b"$err:(license?)" + self.separator)
+            await self.writer.drain()
             return False
 
         return True
@@ -158,6 +159,7 @@ async def send_message(self, message: str) -> None:
         """Send the given message to the socket."""
         assert self.writer is not None
         self.writer.write(message.encode() + self.separator)
+        await self.writer.drain()
 
     async def received(
         self, message: bytes | str, timeout: int = 5, remove: bool = True