diff --git a/include/polarssl/padlock.h b/include/polarssl/padlock.h
index 99ecfee..722c9b2 100644
--- a/include/polarssl/padlock.h
+++ b/include/polarssl/padlock.h
@@ -86,7 +86,7 @@
  */
 int padlock_xcryptcbc( aes_context *ctx,
                        int mode,
-                       int length,
+                       size_t length,
                        unsigned char iv[16],
                        const unsigned char *input,
                        unsigned char *output );