diff --git a/runtime/mount/python_lib/lplots/widgets/dataframe.py b/runtime/mount/python_lib/lplots/widgets/dataframe.py
index b25a295b..c658af05 100644
--- a/runtime/mount/python_lib/lplots/widgets/dataframe.py
+++ b/runtime/mount/python_lib/lplots/widgets/dataframe.py
@@ -30,12 +30,12 @@ def _value(self, key: str | None) -> Any | None:
     @property
     def value(self) -> Any | None:
         key = self._select.value
-        assert isinstance(key, str)
+        assert isinstance(key, str | None)
         return self._value(key)
 
     def sample(self) -> Any | None:
-        key = self._select.sample
-        assert isinstance(key, str)
+        key = self._select.sample()
+        assert isinstance(key, str | None)
         return self._value(key)