diff --git a/lib/nghttp2_option.c b/lib/nghttp2_option.c
index 2201ec6e..fd665112 100644
--- a/lib/nghttp2_option.c
+++ b/lib/nghttp2_option.c
@@ -70,6 +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] = (uint8_t)(
-      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 << (type & 0x7)));
 }
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 6be3c631..6f3843c8 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -5345,7 +5345,7 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
 
         if (!session->callbacks.unpack_extension_callback ||
             (session->user_recv_ext_types[iframe->frame.hd.type / 8] &
-             (1 << (7 - (iframe->frame.hd.type & 0x7)))) == 0) {
+             (1 << (iframe->frame.hd.type & 0x7))) == 0) {
           /* Silently ignore unknown frame type. */
 
           busy = 1;
diff --git a/lib/nghttp2_session.h b/lib/nghttp2_session.h
index 0bf12db5..c319c15e 100644
--- a/lib/nghttp2_session.h
+++ b/lib/nghttp2_session.h
@@ -306,10 +306,11 @@ struct nghttp2_session {
      WINDOW_UPDATE is not queued. */
   uint8_t window_update_queued;
   /* Bitfield of extension frame types that application is willing to
-     receive.  First most significant 10 bits are standard frame types
-     and not used.  If bit is set, it indicates that incoming frame
-     with that type is passed to user defined callbacks, otherwise
-     they are ignored. */
+     receive.  To designate the bit of given frame type i, use
+     user_recv_ext_types[i / 8] & (1 << (i & 0x7)).  First 10 frame
+     types are standard frame types and not used in this bitfield.  If
+     bit is set, it indicates that incoming frame with that type is
+     passed to user defined callbacks, otherwise they are ignored. */
   uint8_t user_recv_ext_types[32];
 };