diff --git a/httpcore/_backends/base.py b/httpcore/_backends/base.py
index cf55c8b1..1318478f 100644
--- a/httpcore/_backends/base.py
+++ b/httpcore/_backends/base.py
@@ -97,5 +97,8 @@ async def connect_unix_socket(
     ) -> AsyncNetworkStream:
         raise NotImplementedError()  # pragma: nocover
 
+    async def aclose(self) -> None:
+        raise NotImplementedError()  # pragma: nocover
+
     async def sleep(self, seconds: float) -> None:
         raise NotImplementedError()  # pragma: nocover