diff --git a/packages/sanity/src/core/preview/utils/portableText.ts b/packages/sanity/src/core/preview/utils/portableText.ts
index f235d3c5a0e..fa8938ffaee 100644
--- a/packages/sanity/src/core/preview/utils/portableText.ts
+++ b/packages/sanity/src/core/preview/utils/portableText.ts
@@ -1,17 +1,17 @@
-import {Block, isBlock, isSpan} from '@sanity/types'
+import {isPortableTextTextBlock, isPortableTextSpan, PortableTextBlock} from '@sanity/types'
 
-export function isPortableTextArray(blocks: unknown): blocks is Block[] {
-  return Array.isArray(blocks) && (blocks.length === 0 || blocks.some(isBlock))
+export function isPortableTextPreviewValue(value: unknown): value is PortableTextBlock[] {
+  return Array.isArray(value) && (value.length === 0 || value.some(isPortableTextTextBlock))
 }
 
-export function extractTextFromBlocks(blocks: Block[]): string {
-  const firstBlock = blocks.find(isBlock)
+export function extractTextFromBlocks(blocks: unknown): string {
+  const firstBlock = Array.isArray(blocks) && blocks.find(isPortableTextTextBlock)
   if (!firstBlock || !firstBlock.children) {
     return ''
   }
 
   return firstBlock.children
-    .filter(isSpan)
+    .filter(isPortableTextSpan)
     .map((span) => span.text)
     .join('')
 }
diff --git a/packages/sanity/src/core/preview/utils/prepareForPreview.ts b/packages/sanity/src/core/preview/utils/prepareForPreview.ts
index e23ffba3fb4..29f3e5c83d5 100644
--- a/packages/sanity/src/core/preview/utils/prepareForPreview.ts
+++ b/packages/sanity/src/core/preview/utils/prepareForPreview.ts
@@ -9,7 +9,7 @@ import {debounce, flatten, get, isPlainObject, pick, uniqBy} from 'lodash'
 import {INVALID_PREVIEW_FALLBACK} from '../constants'
 import {PreviewableType} from '../types'
 import {isRecord} from '../../util'
-import {isPortableTextArray, extractTextFromBlocks} from './portableText'
+import {extractTextFromBlocks, isPortableTextPreviewValue} from './portableText'
 import {keysOf} from './keysOf'
 
 const PRESERVE_KEYS = ['_id', '_type', '_upload', '_createdAt', '_updatedAt']
@@ -190,7 +190,7 @@ function defaultPrepare(value: SelectedValue) {
     const val = value[fieldName]
     return {
       ...acc,
-      [fieldName]: isPortableTextArray(val) ? extractTextFromBlocks(val) : value[fieldName],
+      [fieldName]: isPortableTextPreviewValue(val) ? extractTextFromBlocks(val) : val,
     }
   }, {})
 }