diff --git a/tatsu/contexts.py b/tatsu/contexts.py
index 06458c41..f5d0f962 100644
--- a/tatsu/contexts.py
+++ b/tatsu/contexts.py
@@ -569,6 +569,7 @@ def rule(self):
 
     @property
     def memokey(self):
+        self.tokenizer.eat_whitespace()
         return MemoKey(self._pos, self.rule, self.substate)
 
     def _memoize(self, key, memo):
diff --git a/tatsu/tokenizing.py b/tatsu/tokenizing.py
index 918f0821..bc7077f5 100644
--- a/tatsu/tokenizing.py
+++ b/tatsu/tokenizing.py
@@ -40,6 +40,9 @@ def current(self):
     def token(self):
         return self.current
 
+    def eat_whitespace(self):
+        raise NotImplementedError
+
     def next(self):
         raise NotImplementedError