diff --git a/src/encoder.test.ts b/src/encoder.test.ts
index c7d2d4e..8c7e723 100644
--- a/src/encoder.test.ts
+++ b/src/encoder.test.ts
@@ -1,7 +1,7 @@
 import {describe, expect, test} from 'vitest'
 
 import testDescriptors from './__fixtures__/descriptors.json'
-import {encode, type Encoded} from './encoder'
+import {type EncodableObject, encode, type Encoded} from './encoder'
 
 describe('Fixtures', () => {
   test.each(
@@ -13,3 +13,30 @@ describe('Fixtures', () => {
     expect(encoded.id).toBe(id)
   })
 })
+
+describe('Regressions', () => {
+  test('invalid descriptor ID with rewriteMap', () => {
+    const makeTypeDef = (field: EncodableObject) => ({
+      name: 'species',
+      typeDef: {
+        hello: undefined,
+        extends: 'document',
+        title: 'Species',
+        fields: [field],
+      },
+    })
+
+    const sourceField = {type: 'hello', name: 'world'}
+    const hoisted = {__type: 'hoisted', key: 'book.author._ref'}
+
+    const withSource = makeTypeDef(sourceField)
+    const rewriteMap = new Map()
+    rewriteMap.set(sourceField, hoisted)
+    const result = encode('sanity.schema.namedType', withSource, {rewriteMap})
+
+    const withHoisted = makeTypeDef(hoisted)
+    const manuallyRewritten = encode('sanity.schema.namedType', withHoisted)
+
+    expect(result).toStrictEqual(manuallyRewritten)
+  })
+})
diff --git a/src/encoder.ts b/src/encoder.ts
index 72cefc7..63a2910 100644
--- a/src/encoder.ts
+++ b/src/encoder.ts
@@ -127,6 +127,7 @@ class IDEncoder {
 
       for (const [key, field] of entries) {
         if (field === undefined) {
+          idx++
           continue
         }