diff --git a/src/mnemonic/mnemonic.py b/src/mnemonic/mnemonic.py
index caa4ffb..3fdb687 100644
--- a/src/mnemonic/mnemonic.py
+++ b/src/mnemonic/mnemonic.py
@@ -165,8 +165,9 @@ def to_entropy(self, words: list[str] | str) -> bytearray:
         wordindex = 0
         for word in words:
             # Find the words index in the wordlist
-            ndx = self.wordlist.index(self.normalize_string(word))
-            if ndx < 0:
+            try:
+                ndx = self.wordlist.index(self.normalize_string(word))
+            except ValueError:
                 raise LookupError('Unable to find "%s" in word list.' % word)
             # Set the next 11 bits to the value of the index.
             for ii in range(11):