diff --git a/include/picotls.h b/include/picotls.h
index fc88a14..3476a52 100644
--- a/include/picotls.h
+++ b/include/picotls.h
@@ -493,7 +493,7 @@
     /**
      * if ESNI was used
      */
-    uint8_t esni : 1;
+    unsigned int esni : 1;
 } ptls_on_client_hello_parameters_t;
 
 /**