diff --git a/py/builtinevex.c b/py/builtinevex.c
index 89fcc2f..d9a3833 100644
--- a/py/builtinevex.c
+++ b/py/builtinevex.c
@@ -78,7 +78,7 @@
     (void)n_args;
 
     // get the source
-    mp_uint_t str_len;
+    size_t str_len;
     const char *str = mp_obj_str_get_data(args[0], &str_len);
 
     // get the filename
@@ -128,7 +128,7 @@
     }
     #endif
 
-    mp_uint_t str_len;
+    size_t str_len;
     const char *str = mp_obj_str_get_data(args[0], &str_len);
 
     // create the lexer