diff --git a/dedupe/canopy_index.py b/dedupe/canopy_index.py
index 66e926b3..da3eb995 100644
--- a/dedupe/canopy_index.py
+++ b/dedupe/canopy_index.py
@@ -4,8 +4,8 @@
 import math
 from typing import Iterable
 
-import numpy
 from BTrees.Length import Length
+from numpy import log
 from zope.index.text.cosineindex import CosineIndex
 from zope.index.text.lexicon import Lexicon
 from zope.index.text.setops import mass_weightedUnion
@@ -37,7 +37,7 @@ def initSearch(self) -> None:
                 docs = bucket(docs)
                 self.index._wordinfo[wid] = docs
 
-            idf = numpy.log1p(N / len(docs))
+            idf = 1 + log((1 + N) / (1 + len(docs)))
             term = self.lexicon._words[wid]
 
             self._wids_dict[term] = (wid, idf)