diff --git a/src/nacl/secret.py b/src/nacl/secret.py
index 6ae85876133c5cc1d47144d336ea0d211fc388b5..b81e7ea8ad8abddbc337ed2d2cfd6e16ef564c92 100644
--- a/src/nacl/secret.py
+++ b/src/nacl/secret.py
@@ -49,7 +49,7 @@ class SecretBox(encoding.Encodable, StringFixer, object):
         if len(key) != self.KEY_SIZE:
             raise ValueError(
                 "The key must be exactly %s bytes long" %
-                nacl.lib.crypto_secretbox_KEYBYTES,
+                self.KEY_SIZE,
             )
 
         self._key = key