diff --git a/lib/schema/uuid.js b/lib/schema/uuid.js
index b621fa942d8..6b5793fd3ee 100644
--- a/lib/schema/uuid.js
+++ b/lib/schema/uuid.js
@@ -67,7 +67,7 @@ function stringToBinary(uuidStr) {
 function binaryToString(uuidBin) {
   // i(hasezoey) dont quite know why, but "uuidBin" may sometimes also be the already processed string
   let hex;
-  if (typeof uuidBin !== 'string') {
+  if (typeof uuidBin !== 'string' && uuidBin != null) {
     hex = binary2hex(uuidBin);
     const uuidStr = hex.substring(0, 8) + '-' + hex.substring(8, 8 + 4) + '-' + hex.substring(12, 12 + 4) + '-' + hex.substring(16, 16 + 4) + '-' + hex.substring(20, 20 + 12);
     return uuidStr;
diff --git a/test/schema.uuid.test.js b/test/schema.uuid.test.js
index ee06af9b3b7..8292c233d35 100644
--- a/test/schema.uuid.test.js
+++ b/test/schema.uuid.test.js
@@ -10,6 +10,8 @@ const assert = require('assert');
 const mongoose = start.mongoose;
 const Schema = mongoose.Schema;
 
+const { v4: uuidv4 } = require('uuid');
+
 describe('SchemaUUID', function() {
   let Model;
   let TestSchema;
@@ -101,6 +103,32 @@ describe('SchemaUUID', function() {
     assert.strictEqual(foundDocAll[0].y[2], '5b544b71-8988-422b-a4df-bf691939fe4e');
   });
 
+  it('should not convert to string nullish UUIDs (gh-13032)', async function() {
+    const schema = new Schema({
+      _id: {
+        type: Schema.Types.UUID,
+        default: uuidv4(),
+        immutable: true
+      },
+      name: {
+        type: String,
+        required: true
+      },
+      organization: {
+        type: Schema.Types.UUID,
+        ref: 'Organization',
+        index: true
+      }
+    }, { _id: false });
+
+    const Test = db.model('gh_13032', schema);
+
+    const { name, organization } = await Test.create({ name: 'test' });
+
+    assert.equal(name, 'test');
+    assert.equal(organization, undefined);
+  });
+
   // the following are TODOs based on SchemaUUID.prototype.$conditionalHandlers which are not tested yet
   it('should work with $bits* operators');
   it('should work with $all operator');