diff --git a/beangulp/cache.py b/beangulp/cache.py
index 155cdcf..ab2494c 100644
--- a/beangulp/cache.py
+++ b/beangulp/cache.py
@@ -19,8 +19,8 @@
 
 import chardet
 
-from beancount.utils import defdict
 from beangulp import mimetypes
+from beangulp import utils
 
 # NOTE: See get_file() at the end of this file to create instances of FileMemo.
 
@@ -155,7 +155,7 @@ def get_file(filename):
     return _CACHE[filename]
 
 
-_CACHE = defdict.DefaultDictWithKey(_FileMemo)
+_CACHE = utils.DefaultDictWithKey(_FileMemo)
 
 
 def cache(func=None, *, key=None):
diff --git a/beangulp/utils.py b/beangulp/utils.py
index fa6c022..9567ee3 100644
--- a/beangulp/utils.py
+++ b/beangulp/utils.py
@@ -2,6 +2,7 @@
 from os import path
 from typing import Iterator, Sequence, Union, Set, Optional, Dict
 import datetime
+import collections
 import decimal
 import hashlib
 import logging
@@ -13,6 +14,17 @@
 from beangulp import mimetypes
 
 
+class DefaultDictWithKey(collections.defaultdict):
+    """A version of defaultdict whose factory accepts the key as an argument.
+    Note: collections.defaultdict would be improved by supporting this directly,
+    this is a common occurrence.
+    """
+
+    def __missing__(self, key):
+        self[key] = value = self.default_factory(key)
+        return value
+
+
 def getmdate(filepath: str) -> datetime.date:
     """Return file modification date."""
     mtime = path.getmtime(filepath)
diff --git a/beangulp/utils_test.py b/beangulp/utils_test.py
index 418c96c..1f323a7 100644
--- a/beangulp/utils_test.py
+++ b/beangulp/utils_test.py
@@ -5,7 +5,7 @@
 import os
 import types
 import unittest
-
+from unittest import mock
 from shutil import rmtree
 from tempfile import mkdtemp
 
@@ -111,3 +111,14 @@ def test_idify(self):
         )
         self.assertEqual("A____B.pdf", utils.idify("A____B_._pdf"))
 
+
+class TestDefDictWithKey(unittest.TestCase):
+    def test_defdict_with_key(self):
+        factory = mock.MagicMock()
+        testdict = utils.DefaultDictWithKey(factory)
+
+        testdict["a"]
+        testdict["b"]
+        self.assertEqual(2, len(factory.mock_calls))
+        self.assertEqual(("a",), factory.mock_calls[0][1])
+        self.assertEqual(("b",), factory.mock_calls[1][1])