diff --git a/library/x509_crt.c b/library/x509_crt.c
index b7c73df..4b831ae 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -988,7 +988,7 @@
     p = filename + len;
     filename[len++] = '*';
 
-    w_ret = MultiByteToWideChar( CP_ACP, 0, filename, len, szDir,
+    w_ret = MultiByteToWideChar( CP_ACP, 0, filename, (int)len, szDir,
                                  MAX_PATH - 3 );
     if( w_ret == 0 )
         return( POLARSSL_ERR_X509_BAD_INPUT_DATA );