diff --git a/crossfit/data/dataframe/core.py b/crossfit/data/dataframe/core.py
index 9ec969c7..341bffa0 100644
--- a/crossfit/data/dataframe/core.py
+++ b/crossfit/data/dataframe/core.py
@@ -14,7 +14,7 @@
 
 from __future__ import annotations
 
-from functools import lru_cache
+from functools import cached_property
 from typing import Callable, List
 
 from crossfit.data.array.conversion import convert_array
@@ -348,8 +348,8 @@ def _(data):
 
 # Fall-back `ArrayBundle` definition
 class ArrayBundle(FrameBackend):
-    @lru_cache
-    def __len__(self):
+    @cached_property
+    def _cached_len(self):
         _len = None
         for k, v in self.data.items():
             if _len is None:
@@ -358,6 +358,9 @@ def __len__(self):
                 raise ValueError(f"Column {k} was length {len(v)}, but expected length {_len}")
         return _len
 
+    def __len__(self):
+        return self._cached_len
+
     @property
     def dtypes(self) -> dict:
         # TODO: Does this work for "all" supported Array types?
@@ -421,7 +424,7 @@ def assign(self, **kwargs):
         data = self.data.copy()
         for k, v in kwargs.items():
             if self.columns and len(v) != len(self):
-                raise ValueError(f"Column {k} was length {len(v)}, but expected length {len(self)}")
+                raise ValueError(f"Column {k} was length {len(v)} but expected length {len(self)}")
         data.update(**kwargs)
         return self.__class__(data)