diff --git a/lib/nghttp2_option.c b/lib/nghttp2_option.c
index 81ae6ba7..2201ec6e 100644
--- a/lib/nghttp2_option.c
+++ b/lib/nghttp2_option.c
@@ -70,5 +70,6 @@ void nghttp2_option_set_user_recv_extension_type(nghttp2_option *option,
   }
 
   option->opt_set_mask |= NGHTTP2_OPT_USER_RECV_EXT_TYPES;
-  option->user_recv_ext_types[type / 8] |= 1 << (7 - (type & 0x7));
+  option->user_recv_ext_types[type / 8] = (uint8_t)(
+      option->user_recv_ext_types[type / 8] | (1 << (7 - (type & 0x7))));
 }