diff --git a/include/quicly.h b/include/quicly.h
index d021e1ab..dea5e1d9 100644
--- a/include/quicly.h
+++ b/include/quicly.h
@@ -836,7 +836,7 @@ struct st_quicly_stream_t {
          */
         struct {
             quicly_sender_state_t sender_state;
-            uint16_t error_code;
+            uint64_t error_code;
         } stop_sending;
         /**
          * reset_stream
@@ -846,7 +846,7 @@ struct st_quicly_stream_t {
              * STATE_NONE until RST is generated
              */
             quicly_sender_state_t sender_state;
-            uint16_t error_code;
+            uint64_t error_code;
         } reset_stream;
         /**
          * sends receive window updates to remote peer
diff --git a/lib/quicly.c b/lib/quicly.c
index cb09ee79..28a4761f 100644
--- a/lib/quicly.c
+++ b/lib/quicly.c
@@ -310,7 +310,7 @@ struct st_quicly_conn_t {
          * valid if state is CLOSING
          */
         struct {
-            uint16_t error_code;
+            uint64_t error_code;
             uint64_t frame_type; /* UINT64_MAX if application close */
             const char *reason_phrase;
             unsigned long num_packets_received;
@@ -5783,7 +5783,7 @@ static quicly_error_t enter_close(quicly_conn_t *conn, int local_is_initiating,
 
 quicly_error_t initiate_close(quicly_conn_t *conn, quicly_error_t err, uint64_t frame_type, const char *reason_phrase)
 {
-    uint16_t quic_error_code;
+    uint64_t quic_error_code;
 
     if (conn->super.state >= QUICLY_STATE_CLOSING)
         return 0;