diff --git a/asyncpg/pool.py b/asyncpg/pool.py
index e3898d5..0bea17a 100644
--- a/asyncpg/pool.py
+++ b/asyncpg/pool.py
@@ -10,6 +10,7 @@ import functools
 import inspect
 import logging
 import time
+from typing import Optional
 import warnings
 
 from . import compat
@@ -384,7 +385,7 @@ class Pool:
         self._holders = []
         self._initialized = False
         self._initializing = False
-        self._queue = None
+        self._queue: Optional[asyncio.LifoQueue[PoolConnectionHolder]] = None
 
         self._connection_class = connection_class
         self._record_class = record_class
@@ -842,11 +843,12 @@ class Pool:
         """
         return PoolAcquireContext(self, timeout)
 
-    async def _acquire(self, timeout):
-        async def _acquire_impl():
-            ch = await self._queue.get()  # type: PoolConnectionHolder
+    async def _acquire(self, timeout: Optional[float]) -> PoolConnectionProxy:
+        async def _acquire_impl() -> PoolConnectionProxy:
+            assert self._queue is not None, "Pool is not initialized"
+            ch = await self._queue.get()
             try:
-                proxy = await ch.acquire()  # type: PoolConnectionProxy
+                proxy = await ch.acquire()
             except (Exception, asyncio.CancelledError):
                 self._queue.put_nowait(ch)
                 raise
@@ -1015,10 +1017,10 @@ class PoolAcquireContext:
     def __init__(self, pool, timeout):
         self.pool = pool
         self.timeout = timeout
-        self.connection = None
+        self.connection: Optional[PoolConnectionProxy] = None
         self.done = False
 
-    async def __aenter__(self):
+    async def __aenter__(self) -> PoolConnectionProxy:
         if self.connection is not None or self.done:
             raise exceptions.InterfaceError('a connection is already acquired')
         self.connection = await self.pool._acquire(self.timeout)