diff --git a/README.md b/README.md
index 7c06b4b..72756d5 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,8 @@ to the `require` section of your `composer.json` file.
 
 ## Migrations example
 
+Warning! `depth` attribute can not be unsigned!
+
 Single tree migration:
 
 ```php