diff --git a/src/middleware/entityIssueDetails.middleware.js b/src/middleware/entityIssueDetails.middleware.js
index 866431cd4..19ce19707 100644
--- a/src/middleware/entityIssueDetails.middleware.js
+++ b/src/middleware/entityIssueDetails.middleware.js
@@ -26,7 +26,17 @@ export const IssueDetailsQueryParams = v.object({
   dataset: v.string(),
   issue_type: v.string(),
   issue_field: v.string(),
-  pageNumber: v.optional(v.pipe(v.string(), v.transform(s => parseInt(s, 10)), v.minValue(1)), '1'),
+  pageNumber: v.optional(
+    v.pipe(
+      v.string(),
+      v.regex(/^\d+$/),
+      v.transform(s => Number(s)),
+      v.number(),
+      v.integer(),
+      v.minValue(1)
+    ),
+    '1'
+  ),
   resourceId: v.optional(v.string())
 })
 
diff --git a/test/unit/middleware/entityIssueDetails.middleware.test.js b/test/unit/middleware/entityIssueDetails.middleware.test.js
index 7e4704ca0..61dea1d9f 100644
--- a/test/unit/middleware/entityIssueDetails.middleware.test.js
+++ b/test/unit/middleware/entityIssueDetails.middleware.test.js
@@ -1,5 +1,7 @@
 import { describe, it, vi, expect, beforeEach } from 'vitest'
-import { getIssueDetails, getIssueField, prepareEntity, setRecordCount, show404ifNoIssues } from '../../../src/middleware/entityIssueDetails.middleware.js'
+import * as v from 'valibot'
+import { isValiError } from 'valibot'
+import { IssueDetailsQueryParams, getIssueDetails, getIssueField, prepareEntity, setRecordCount, show404ifNoIssues } from '../../../src/middleware/entityIssueDetails.middleware.js'
 import { MiddlewareError } from '../../../src/utils/errors.js'
 
 vi.mock('../../../src/services/performanceDbApi.js')
@@ -255,4 +257,29 @@ describe('issueDetails.middleware.js', () => {
       expect(next).toHaveBeenCalledWith()
     })
   })
+
+  describe('IssueDetailsQueryParams', () => {
+    const validBaseParams = {
+      lpa: 'local-authority:LIV',
+      dataset: 'tree-preservation-zone',
+      issue_type: 'missing associated entity',
+      issue_field: 'tree-preservation-order'
+    }
+
+    it('transforms a valid numeric pageNumber string to a number', () => {
+      const result = v.parse(IssueDetailsQueryParams, { ...validBaseParams, pageNumber: '2' })
+      expect(result.pageNumber).toBe(2)
+    })
+
+    it('throws a ValiError when pageNumber is not numeric', () => {
+      let thrown
+      try {
+        v.parse(IssueDetailsQueryParams, { ...validBaseParams, pageNumber: 'TPO335.G19' })
+      } catch (error) {
+        thrown = error
+      }
+      expect(thrown).toBeDefined()
+      expect(isValiError(thrown)).toBe(true)
+    })
+  })
 })