diff --git a/aiomcache/client.py b/aiomcache/client.py
index 78896f4..eeba3f4 100644
--- a/aiomcache/client.py
+++ b/aiomcache/client.py
@@ -410,10 +410,12 @@ async def _incr_decr(
     ) -> Optional[int]:
         cmd = b"%b %b %a\r\n" % (command, key, delta)
         resp = await self._execute_simple_command(conn, cmd)
-        if not resp.isdigit() or resp == const.NOT_FOUND:
+        if resp == const.NOT_FOUND:
+            return None
+        if not resp.isdigit():
             raise ClientException(
                 'Memcached {} command failed'.format(str(command)), resp)
-        return int(resp) if resp.isdigit() else None
+        return int(resp)
 
     @acquire
     async def incr(self, conn: Connection, key: bytes, increment: int = 1) -> Optional[int]:
diff --git a/tests/commands_test.py b/tests/commands_test.py
index ae5768e..32994cb 100644
--- a/tests/commands_test.py
+++ b/tests/commands_test.py
@@ -299,6 +299,9 @@ async def test_incr_errors(mcache: Client) -> None:
     with pytest.raises(ClientException):
         await mcache.incr(key, 3.14)  # type: ignore[arg-type]
 
+    ivalue = await mcache.incr(b'not:' + key, 5)
+    assert ivalue is None
+
 
 async def test_decr(mcache: Client) -> None:
     key, value = b'key:decr:1', b'17'
@@ -324,6 +327,9 @@ async def test_decr_errors(mcache: Client) -> None:
     with pytest.raises(ClientException):
         await mcache.decr(key, 3.14)  # type: ignore[arg-type]
 
+    ivalue = await mcache.decr(b'not:' + key, 6)
+    assert ivalue is None
+
 
 async def test_stats(mcache: Client) -> None:
     stats = await mcache.stats()