diff --git a/CHANGELOG.md b/CHANGELOG.md
index dcd02fe..5e1c5cc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,9 +31,8 @@
   `collection`, `id` and `occurrenceKey` directly where they rode the opaque carry.
 - Fix an image inside another image's description: it flattens into the alt text, where it was
   refused.
-- Fix a cyclic document, in which a node or an attribute value holds itself: `adfToMarkdown` and
-  `adfToPlainMarkdown` refuse it as `not-an-adf-document` and `isAdfDocument` returns `false`,
-  where all three hung.
+- Fix a cyclic document, in which an object holds itself: `adfToMarkdown` and `adfToPlainMarkdown`
+  refuse it as `not-an-adf-document` and `isAdfDocument` returns `false`, where all three hung.
 
 ## 0.1.0
 
diff --git a/README.md b/README.md
index 40826c5..71cde23 100644
--- a/README.md
+++ b/README.md
@@ -185,7 +185,7 @@ 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, a node or attribute value 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 | guard the boundary you receive JSON at with `isAdfDocument`; the message names the branch that refused |
 | `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
diff --git a/docs/decisions.md b/docs/decisions.md
index 789633c..415f3a2 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -385,9 +385,9 @@ input reads `message`.
   it names the escape that unclaims the form claimed: `\!adf:` for a directive, block line and
   inline alike, `\|` for every pipe row.
 - `not-an-adf-document` carries the document's own path throughout: seven of the guard's nine
-  branches read the document's own shape, and threading a path to the other two — a malformed node,
-  or an object holding itself, anywhere in the tree — wants the manual stack §Nothing recurses unbounded forces. The message
-  names the violation instead.
+  branches read the document's own shape, and threading a path to the other two, which look anywhere
+  in the tree — a malformed node, or an object holding itself — wants the manual stack §Nothing
+  recurses unbounded forces. The message names the violation instead.
 
 ## Publish on a version bump
 
diff --git a/src/adf/document.test.ts b/src/adf/document.test.ts
index bb9ccab..af13f46 100644
--- a/src/adf/document.test.ts
+++ b/src/adf/document.test.ts
@@ -84,7 +84,7 @@ test("reads a node's siblings as a walk rather than as one call's arguments", ()
 })
 
 test('refuses a node or an attribute value holding itself, and accepts one shared', () => {
-  const holdsItself = 'an ADF document is a tree: a node or an attribute value holds itself'
+  const holdsItself = 'an ADF document is a tree: an object in it holds itself'
   const paragraph: { content: unknown[]; type: string } = { content: [], type: 'paragraph' }
   paragraph.content.push(paragraph)
   assert.equal(fault({ content: [paragraph], type: 'doc', version: 1 }), holdsItself)
@@ -96,6 +96,18 @@ test('refuses a node or an attribute value holding itself, and accepts one share
   assert.equal(fault({ content: [{ attrs, type: 'paragraph' }], type: 'doc', version: 1 }), holdsItself)
   assert.equal(fault({ content: [{ marks: [{ attrs, type: 'link' }], text: 'x', type: 'text' }], type: 'doc', version: 1 }), holdsItself)
   assert.equal(isAdfDocument({ content: [{ attrs, type: 'paragraph' }], type: 'doc', version: 1 }), false)
+  const notANode = "an ADF document's content holds ADF nodes: one of them is not"
+  const inherits: { content?: unknown[] } = {}
+  const heir: Record<string, unknown> = Object.create(inherits)
+  heir['type'] = 'paragraph'
+  inherits.content = [heir]
+  assert.equal(fault({ content: [heir], type: 'doc', version: 1 }), notANode)
+  const hidden: Record<string, unknown> = { type: 'paragraph' }
+  Object.defineProperty(hidden, 'content', { value: [hidden] })
+  assert.equal(fault({ content: [hidden], type: 'doc', version: 1 }), notANode)
+  const link: Record<string, unknown> = Object.create({ attrs })
+  link['type'] = 'link'
+  assert.equal(fault({ content: [{ marks: [link], text: 'x', type: 'text' }], type: 'doc', version: 1 }), notANode)
   const bold = { type: 'strong' }
   const text = { marks: [bold], text: 'x', type: 'text' }
   const shared = { content: [text, text], type: 'paragraph' }
diff --git a/src/adf/document.ts b/src/adf/document.ts
index 3e138eb..7c4060b 100644
--- a/src/adf/document.ts
+++ b/src/adf/document.ts
@@ -43,7 +43,7 @@ export function adfDocumentFault(value: unknown): ConvertFault | undefined {
   const held: unknown = value['content']
   if (!Array.isArray(held)) return notADocument(`an ADF document's content is an array: found ${describe(held)}`)
   const content: readonly unknown[] = held
-  if (holdsItself(content)) return notADocument('an ADF document is a tree: a node or an attribute value holds itself')
+  if (holdsItself(content)) return notADocument('an ADF document is a tree: an object in it holds itself')
   if (!isNodeArray(content)) return notADocument("an ADF document's content holds ADF nodes: one of them is not")
   return nestingFault(content)
 }
@@ -106,7 +106,7 @@ export function isAdfNode(value: unknown): value is AdfNode {
 }
 
 export function isAdfMark(value: unknown): value is AdfMark {
-  if (!isRecord(value) || !holdsOnly(value, markKeys)) return false
+  if (!isRecord(value) || !holdsOnly(value, markKeys) || !ownsWhatItHolds(value, markKeys)) return false
   if (typeof value['type'] !== 'string') return false
   return !('attrs' in value) || isAttributes(value['attrs'])
 }
@@ -123,15 +123,15 @@ export function nodeMarks(node: { marks?: AdfMark[] }): readonly AdfMark[] {
   return node.marks ?? []
 }
 
-// Ancestors alone: a tree may share an object, but an object holding itself walks the guards forever.
+// Tracks ancestors alone: a tree may share an object, but an object holding itself walks the guards forever.
 function holdsItself(root: object): boolean {
   const pending = [{ depth: 0, item: root }]
   const ancestors: object[] = []
-  const onPath = new Set<object>()
+  const onPath = new Set<unknown>()
   while (pending.length > 0) {
     const entry = pending.pop()
     if (entry === undefined) continue
-    for (const left of ancestors.splice(entry.depth)) onPath.delete(left)
+    while (ancestors.length > entry.depth) onPath.delete(ancestors.pop())
     if (onPath.has(entry.item)) return true
     ancestors.push(entry.item)
     onPath.add(entry.item)
@@ -144,7 +144,7 @@ function isNodeArray(value: readonly unknown[]): value is readonly AdfNode[] {
   const pending: unknown[] = [...value]
   while (pending.length > 0) {
     const node = pending.pop()
-    if (!isRecord(node) || !holdsOnly(node, nodeKeys)) return false
+    if (!isRecord(node) || !holdsOnly(node, nodeKeys) || !ownsWhatItHolds(node, nodeKeys)) return false
     if (typeof node['type'] !== 'string') return false
     if ('attrs' in node && !isAttributes(node['attrs'])) return false
     if ('marks' in node && !isArrayOf(node['marks'], isAdfMark)) return false
@@ -219,6 +219,11 @@ function extraKey(value: Record<string, unknown>, keys: readonly string[]): stri
   return Object.keys(value).find((key) => !keys.includes(key))
 }
 
+// holdsItself walks own enumerable keys alone, so a guard reading an inherited one could walk a cycle forever.
+function ownsWhatItHolds(value: Record<string, unknown>, keys: readonly string[]): boolean {
+  return keys.every((key) => !(key in value) || Object.prototype.propertyIsEnumerable.call(value, key))
+}
+
 function holdsOnly(value: Record<string, unknown>, keys: readonly string[]): boolean {
   return extraKey(value, keys) === undefined
 }