diff --git a/packages/core/src/block/content/atxHeadingContent/__tests__/enterHandler.spec.ts b/packages/core/src/block/content/atxHeadingContent/__tests__/enterHandler.spec.ts
new file mode 100644
index 00000000..005a6d5a
--- /dev/null
+++ b/packages/core/src/block/content/atxHeadingContent/__tests__/enterHandler.spec.ts
@@ -0,0 +1,102 @@
+// @vitest-environment happy-dom
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { ScrollPage } from '../../../scrollPage';
+import AtxHeadingContent from '../index';
+
+// Regression: pressing Enter at the start of an ATX heading (e.g. before
+// the leading `#`) inserts a new empty paragraph above the heading, but
+// the handler used to forget to call `event.preventDefault()`. The
+// browser's default Enter behavior then ran on the contenteditable —
+// splitting the heading's `<span class="mu-content">` or cloning it —
+// producing orphan `mu-content` nodes that were NOT linked back to a
+// block via `BLOCK_DOM_PROPERTY`. A subsequent click would resolve to
+// such an orphan and `Selection.getSelection` crashed with
+// `Cannot read properties of undefined (reading 'path')`.
+//
+// We exercise the handler directly with a structurally-typed `this` so
+// no Muya bootstrap is required.
+
+interface FakeAtxContent {
+    text: string;
+    cursor: { start: number; end: number };
+    parent: {
+        meta: { level: number };
+        parent: { insertBefore: (block: unknown, ref: unknown) => void };
+    };
+    muya: unknown;
+    getCursor: () => { start: { offset: number }; end: { offset: number } };
+    setCursor: (start: number, end: number, _selected?: boolean) => void;
+}
+
+function makeFakeContent(level: number, cursorAt: number): FakeAtxContent {
+    return {
+        text: '# Headings',
+        cursor: { start: cursorAt, end: cursorAt },
+        parent: {
+            meta: { level },
+            parent: { insertBefore: vi.fn() },
+        },
+        muya: {},
+        getCursor() {
+            return {
+                start: { offset: this.cursor.start },
+                end: { offset: this.cursor.end },
+            };
+        },
+        setCursor(start, end) {
+            this.cursor = { start, end };
+        },
+    };
+}
+
+function makeKeyEvent() {
+    return {
+        preventDefault: vi.fn(),
+        stopPropagation: vi.fn(),
+    } as unknown as KeyboardEvent;
+}
+
+describe('atxHeadingContent.enterHandler — prevents default when inserting paragraph above heading', () => {
+    afterEach(() => {
+        vi.restoreAllMocks();
+    });
+
+    it('calls preventDefault when Enter is pressed at offset 0 in an H1', () => {
+        // Stub ScrollPage.loadBlock so the handler can synthesize a new
+        // paragraph block without bootstrapping the registry.
+        const fakeParagraphBlock = {};
+        const create = vi.fn().mockReturnValue(fakeParagraphBlock);
+        vi.spyOn(ScrollPage, 'loadBlock').mockReturnValue({ create } as never);
+
+        const content = makeFakeContent(1, 0);
+        const event = makeKeyEvent();
+
+        AtxHeadingContent.prototype.enterHandler.call(
+            content as unknown as AtxHeadingContent,
+            event,
+        );
+
+        expect(event.preventDefault).toHaveBeenCalledTimes(1);
+        // Sanity: the paragraph insert path actually ran.
+        expect(content.parent.parent.insertBefore).toHaveBeenCalledTimes(1);
+    });
+
+    it('calls preventDefault when Enter is pressed right after the `#` in an H1 (offset 1)', () => {
+        const fakeParagraphBlock = {};
+        const create = vi.fn().mockReturnValue(fakeParagraphBlock);
+        vi.spyOn(ScrollPage, 'loadBlock').mockReturnValue({ create } as never);
+
+        // For level=1, the guard is `offset <= level + 1` (=2), so offset
+        // 1 still routes through the insert-paragraph branch.
+        const content = makeFakeContent(1, 1);
+        const event = makeKeyEvent();
+
+        AtxHeadingContent.prototype.enterHandler.call(
+            content as unknown as AtxHeadingContent,
+            event,
+        );
+
+        expect(event.preventDefault).toHaveBeenCalledTimes(1);
+        expect(content.parent.parent.insertBefore).toHaveBeenCalledTimes(1);
+    });
+});
diff --git a/packages/core/src/block/content/atxHeadingContent/index.ts b/packages/core/src/block/content/atxHeadingContent/index.ts
index 6efb4d45..86edef5b 100644
--- a/packages/core/src/block/content/atxHeadingContent/index.ts
+++ b/packages/core/src/block/content/atxHeadingContent/index.ts
@@ -35,6 +35,15 @@ class AtxHeadingContent extends Format {
         const { level } = this.parent!.meta;
 
         if (start.offset === end.offset && start.offset <= level + 1) {
+            // Without preventDefault the browser still runs its native
+            // Enter behavior on the contenteditable after we insert the
+            // new paragraph, which can split or clone the heading's
+            // `mu-content` span. Orphaned spans lack BLOCK_DOM_PROPERTY,
+            // so a later click crashes Selection.getSelection at
+            // `anchorBlock.path`.
+            event.preventDefault();
+            event.stopPropagation();
+
             const newNodeState = {
                 name: 'paragraph',
                 text: '',
diff --git a/packages/core/src/selection/index.ts b/packages/core/src/selection/index.ts
index 87519c40..25bf5a2e 100644
--- a/packages/core/src/selection/index.ts
+++ b/packages/core/src/selection/index.ts
@@ -226,8 +226,13 @@ class Selection {
         if (!anchorDomNode || !focusDomNode)
             return null;
 
-        const anchorBlock = anchorDomNode[BLOCK_DOM_PROPERTY] as Content;
-        const focusBlock = focusDomNode[BLOCK_DOM_PROPERTY] as Content;
+        const anchorBlock = anchorDomNode[BLOCK_DOM_PROPERTY] as Content | undefined;
+        const focusBlock = focusDomNode[BLOCK_DOM_PROPERTY] as Content | undefined;
+        // An `mu-content` span cloned by the browser's native edit
+        // behavior is not linked back to a block. Bail out instead of
+        // crashing — the caller treats null the same as "no selection".
+        if (!anchorBlock || !focusBlock)
+            return null;
         const anchorPath = anchorBlock.path;
         const focusPath = focusBlock.path;