diff --git a/src/mnemonic/mnemonic.py b/src/mnemonic/mnemonic.py
index 9457f9d..2896773 100644
--- a/src/mnemonic/mnemonic.py
+++ b/src/mnemonic/mnemonic.py
@@ -141,7 +141,7 @@ def to_entropy(self, words: Union[List[str], str]) -> bytearray:
         wordindex = 0
         for word in words:
             # Find the words index in the wordlist
-            ndx = self.wordlist.index(word)
+            ndx = self.wordlist.index(self.normalize_string(word))
             if ndx < 0:
                 raise LookupError('Unable to find "%s" in word list.' % word)
             # Set the next 11 bits to the value of the index.