diff --git a/guides/providing-services.md b/guides/providing-services.md
index 5cce90960..4dd24d958 100644
--- a/guides/providing-services.md
+++ b/guides/providing-services.md
@@ -765,8 +765,11 @@ Elements annotated with `@readonly`, as well as [_calculated elements_](../cds/c
 By default [`virtual` elements](../cds/cdl#virtual-elements) are also _calculated_.
 ::: tip
 The same applies for fields with the [OData Annotations](../advanced/odata#annotations) `@FieldControl.ReadOnly` (static), `@Core.Computed`, or `@Core.Immutable` (the latter only on UPDATEs).
+:::
 
- :::
+::: warning Not allowed on keys
+Do not use the `@readonly` annotation on keys in all variants.
+:::
 
 ### `@mandatory`