diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8cb2fa7..c9dbce9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,8 +2,10 @@
 
 ## Unreleased
 
-- Fix an attribute value JSON does not hold — a `Date`, `Map`, `Set`, typed array or class instance —
-  passing `isAdfDocument` and converting as `{}`: it is `not-an-adf-document`.
+- **Breaking:** an attribute value that is not a plain object, array or JSON primitive, such as a
+  `Date`, `Map`, typed array or class instance, makes `isAdfDocument` return `false` and the
+  converters refuse it as `not-an-adf-document`, where it was written as `{}` or its own keys.
+  Build it as JSON holds it: `date.toISOString()`, `Object.fromEntries(map)`, `{ ...instance }`.
 - The README opens with what the package does and for whom, and `package.json` carries search
   keywords.
 - The source moves to [github.com/larvit/adf-codec](https://github.com/larvit/adf-codec), where
diff --git a/README.md b/README.md
index 26d40c9..50770a7 100644
--- a/README.md
+++ b/README.md
@@ -195,7 +195,7 @@ Emitting — `adfToMarkdown`, and `adfToHtml` at `0.2.0`:
 
 | Code | Fires when | What you can do |
 | --- | --- | --- |
-| `not-an-adf-document` | the value handed in is no ADF document — a missing or wrong `type`, a stray key, a node that is not a node, an attribute value JSON does not hold — a `Date`, `Map` or class instance —, an object in the document holding itself | guard the boundary you receive JSON at with `isAdfDocument`; the message names the branch that refused |
+| `not-an-adf-document` | the value handed in is no ADF document — a missing or wrong `type`, a stray key, a node that is not a node, an object in the document holding itself, an attribute value that is not a plain object, array or JSON primitive, such as a `Date` or `Map` | guard the boundary you receive JSON at with `isAdfDocument`; the message names the branch that refused. Build an attribute value as JSON holds it: `date.toISOString()`, `Object.fromEntries(map)`, `{ ...instance }` |
 | `unsupported-document-version` | the document's `version` is not 1 | keep the ADF and pass the document over, or show it read-only; the version is the site's, not yours to change |
 
 Either direction. The emitter's own refusals are in this group — a parse reaches them by asking it