diff --git a/cocotbext/axi/stream.py b/cocotbext/axi/stream.py
index e8e17a5..54b1af6 100644
--- a/cocotbext/axi/stream.py
+++ b/cocotbext/axi/stream.py
@@ -177,6 +177,9 @@ class StreamSink(object):
 
         super().__init__(*args, **kwargs)
 
+        self.ready = None
+        self.valid = None
+
         if self._ready_signal is not None and hasattr(self.bus, self._ready_signal):
             self.ready = getattr(self.bus, self._ready_signal)
             self.ready.setimmediatevalue(0)
@@ -269,6 +272,9 @@ class StreamMonitor(object):
 
         super().__init__(*args, **kwargs)
 
+        self.ready = None
+        self.valid = None
+
         if self._ready_signal is not None and hasattr(self.bus, self._ready_signal):
             self.ready = getattr(self.bus, self._ready_signal)