diff --git a/nmigen/back/pysim.py b/nmigen/back/pysim.py
index 47d84fa..db4a678 100644
--- a/nmigen/back/pysim.py
+++ b/nmigen/back/pysim.py
@@ -53,7 +53,7 @@ class _RHSValueCompiler(ValueTransformer):
         return lambda state: value.value
 
     def on_Signal(self, value):
-        if self.sensitivity:
+        if self.sensitivity is not None:
             self.sensitivity.add(value)
         if self.signal_mode == "curr":
             return lambda state: state.curr[value]