diff --git a/protocol/README.md b/protocol/README.md
index 54aa466..8adcd93 100644
--- a/protocol/README.md
+++ b/protocol/README.md
@@ -22,6 +22,7 @@ Codes in [errors.json](errors.json) fall into these ranges:
 | 40300 | 403 |
 | 40400 | 404 |
 | 40500 | 405 |
+| 40900 | 409 |
 | 41001 | 410 |
 | 42200 | 422 |
 | 42910 | 429 |
diff --git a/protocol/errors.json b/protocol/errors.json
index bfa8a62..568d072 100644
--- a/protocol/errors.json
+++ b/protocol/errors.json
@@ -79,6 +79,8 @@
 
   "40500": "method not allowed",
 
+  "40900": "conflict",
+
   "41001": "push device registration expired",
 
   "42200": "Unprocessable entity",