diff --git a/src/uix/soma/components/words/engine/operations/selection-walkers.ts b/src/uix/soma/components/words/engine/operations/selection-walkers.ts
index d617897a3..30cf47061 100644
--- a/src/uix/soma/components/words/engine/operations/selection-walkers.ts
+++ b/src/uix/soma/components/words/engine/operations/selection-walkers.ts
@@ -67,10 +67,15 @@ export function getTextEntries(doc: WordsDocument): readonly WordsTextEntry[] {
 }
 
 function collectBlockText(
-	block: WordsDocument['children'][number],
+	block: WordsDocument['children'][number] | undefined,
 	path: WordsPath,
 	out: WordsTextEntry[]
 ): void {
+	// Robustness: a cell / column / callout child slot can momentarily be
+	// undefined during an editing transition. Skip it rather than throwing
+	// from the mark-walk — an uncaught error here aborts the command and
+	// leaves the editor unable to edit at all.
+	if (!block) return;
 	switch (block.type) {
 		case 'paragraph':
 		case 'heading':