diff --git a/productivity/mock.py b/productivity/mock.py
index e8e258b..a42bc26 100644
--- a/productivity/mock.py
+++ b/productivity/mock.py
@@ -38,6 +38,7 @@ def __init__(self, address, tag_filepath, timeout=1, *args, **kwargs):
         self._coils = defaultdict(bool)
         self._discrete_inputs = defaultdict(bool)
         self._registers = defaultdict(bytes)
+        self._register_types = ['holding', 'input']
         self._detect_pymodbus_version()
         if self.pymodbus33plus:
             self.client.close = lambda: None
diff --git a/productivity/util.py b/productivity/util.py
index e62ad8b..0137822 100644
--- a/productivity/util.py
+++ b/productivity/util.py
@@ -45,12 +45,11 @@ class AsyncioModbusClient:
     including standard timeouts, async context manager, and queued requests.
     """
 
-    _register_types = ['holding', 'input']
-
     def __init__(self, address, timeout=1):
         """Set up communication parameters."""
         self.ip = address
         self.timeout = timeout
+        self._register_types = ['holding', 'input']
         self._detect_pymodbus_version()
         if self.pymodbus30plus:
             self.client = AsyncModbusTcpClient(address, timeout=timeout)