diff --git a/client/src/features/reading-history/__tests__/reading-history-lib.spec.ts b/client/src/features/reading-history/__tests__/reading-history-lib.spec.ts
index 836279089..04b19a162 100644
--- a/client/src/features/reading-history/__tests__/reading-history-lib.spec.ts
+++ b/client/src/features/reading-history/__tests__/reading-history-lib.spec.ts
@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import type { ReadingHistoryItem } from '@bookorbit/types'
 import { filtersFromQuery, queryFromFilters } from '../composables/useReadingHistoryFilters'
-import { exportFileName, toCsv, toMarkdown } from '../lib/history-export'
+import { canvasToBlob, exportFileName, toCsv, toMarkdown } from '../lib/history-export'
 import { readSpanDays } from '../lib/history-format'
 import { historyRouteForChartClick } from '../lib/chart-month-link'
 import { fitTiles, withAlpha } from '../lib/share-poster'
@@ -117,6 +117,24 @@ describe('reading history export', () => {
     expect(exportFileName('Q3 2026', 'md')).toBe('reading-history-q3-2026.md')
     expect(exportFileName('All time', 'csv')).toBe('reading-history-all-time.csv')
   })
+
+  it('waits for the canvas export callback when toBlob returns undefined', async () => {
+    const blob = new Blob(['poster'], { type: 'image/png' })
+    const canvas = {
+      toBlob(callback: BlobCallback) {
+        queueMicrotask(() => callback(blob))
+      },
+    } as HTMLCanvasElement
+
+    await expect(canvasToBlob(canvas, 'image/png')).resolves.toBe(blob)
+  })
+
+  it('rejects when the canvas is unavailable or produces no data', async () => {
+    const canvas = { toBlob: (callback: BlobCallback) => callback(null) } as HTMLCanvasElement
+
+    await expect(canvasToBlob(null, 'image/png')).rejects.toThrow('Canvas is unavailable')
+    await expect(canvasToBlob(canvas, 'image/png')).rejects.toThrow('Canvas export returned no data')
+  })
 })
 
 describe('reading history helpers', () => {
diff --git a/client/src/features/reading-history/components/ShareImageDialog.vue b/client/src/features/reading-history/components/ShareImageDialog.vue
index 72793e096..ab0fafa97 100644
--- a/client/src/features/reading-history/components/ShareImageDialog.vue
+++ b/client/src/features/reading-history/components/ShareImageDialog.vue
@@ -13,7 +13,7 @@ import { useAuth } from '@/features/auth/composables/useAuth'
 import { useCoverVersions } from '@/features/book/composables/useCoverVersions'
 import { fetchReadingHistoryPage, fetchReadingHistorySummary } from '../api/reading-history.api'
 import { formatHours, formatMonthName } from '../lib/history-format'
-import { saveBlob } from '../lib/history-export'
+import { canvasToBlob, saveBlob } from '../lib/history-export'
 import { POSTER_SIZES, drawPoster, type PosterLook, type PosterShape, type PosterTile } from '../lib/share-poster'
 
 type SharePeriod = 'year' | 'quarter' | 'month'
@@ -240,10 +240,7 @@ watch(books, (list) => {
 watch([books, shape, look, showTitles, showRereads, showRatings, imageVersion, canvas, loading], () => void nextTick(scheduleDraw))
 
 function posterBlob(): Promise<Blob> {
-  return new Promise(
-    (resolve, reject) =>
-      canvas.value?.toBlob((blob) => (blob ? resolve(blob) : reject(new Error('empty'))), 'image/png') ?? reject(new Error('no canvas')),
-  )
+  return canvasToBlob(canvas.value, 'image/png')
 }
 
 function fileName() {
diff --git a/client/src/features/reading-history/lib/history-export.ts b/client/src/features/reading-history/lib/history-export.ts
index 3b04c8379..d60689923 100644
--- a/client/src/features/reading-history/lib/history-export.ts
+++ b/client/src/features/reading-history/lib/history-export.ts
@@ -123,3 +123,11 @@ export function saveBlob(blob: Blob, fileName: string) {
   link.remove()
   setTimeout(() => URL.revokeObjectURL(url), 30_000)
 }
+
+export function canvasToBlob(canvas: HTMLCanvasElement | null, type: string): Promise<Blob> {
+  if (!canvas) return Promise.reject(new Error('Canvas is unavailable'))
+
+  return new Promise((resolve, reject) => {
+    canvas.toBlob((blob) => (blob ? resolve(blob) : reject(new Error('Canvas export returned no data'))), type)
+  })
+}