diff --git a/README.md b/README.md
index 5f51311..adb0b9b 100644
--- a/README.md
+++ b/README.md
@@ -99,6 +99,8 @@ echoes its `data`.
 ## Cache button (mention priming)
 
 Discord clients render `<@id>` as a raw id until the user object is cached locally.
+Omit `cache` (or pass `None` / `[]`) and the button never appears; a non-empty cache
+must have one entry per page or construction raises.
 The cache button fixes that: pass `cache=[...]` with one entry per page, where each
 entry is a string of the user mentions on that page (`"<@111> <@222> <@333>"`).