diff --git a/app/forms/subnet-create.tsx b/app/forms/subnet-create.tsx
index b492202bc..d1953aeee 100644
--- a/app/forms/subnet-create.tsx
+++ b/app/forms/subnet-create.tsx
@@ -6,7 +6,6 @@
  * Copyright Oxide Computer Company
  */
 import { useNavigate } from 'react-router-dom'
-import type { SetRequired } from 'type-fest'
 
 import { useApiMutation, useApiQueryClient, type VpcSubnetCreate } from '@oxide/api'
 
@@ -24,13 +23,14 @@ import { useForm, useVpcSelector } from '~/hooks'
 import { FormDivider } from '~/ui/lib/Divider'
 import { pb } from '~/util/path-builder'
 
-const defaultValues: SetRequired<VpcSubnetCreate, 'customRouter'> = {
+const defaultValues: Required<VpcSubnetCreate> = {
   name: '',
+  description: '',
+  ipv4Block: '',
+  ipv6Block: '',
   // populate the form field with the value corresponding to an undefined custom
   // router on a subnet response
   customRouter: customRouterDataToForm(undefined),
-  description: '',
-  ipv4Block: '',
 }
 
 export function CreateSubnetForm() {
@@ -56,12 +56,15 @@ export function CreateSubnetForm() {
       formType="create"
       resourceName="subnet"
       onDismiss={onDismiss}
-      onSubmit={(body) =>
+      onSubmit={({ name, description, ipv4Block, ipv6Block, customRouter }) =>
         createSubnet.mutate({
           query: vpcSelector,
           body: {
-            ...body,
-            customRouter: customRouterFormToData(body.customRouter),
+            name,
+            description,
+            ipv4Block,
+            ipv6Block: ipv6Block.trim() || undefined,
+            customRouter: customRouterFormToData(customRouter),
           },
         })
       }
diff --git a/app/forms/subnet-edit.tsx b/app/forms/subnet-edit.tsx
index 5c87e9c8b..a8b5863c9 100644
--- a/app/forms/subnet-edit.tsx
+++ b/app/forms/subnet-edit.tsx
@@ -6,7 +6,6 @@
  * Copyright Oxide Computer Company
  */
 import { useNavigate, type LoaderFunctionArgs } from 'react-router-dom'
-import type { SetRequired } from 'type-fest'
 
 import {
   apiQueryClient,
@@ -57,7 +56,7 @@ export function EditSubnetForm() {
     },
   })
 
-  const defaultValues: SetRequired<VpcSubnetUpdate, 'customRouter'> = {
+  const defaultValues: Required<VpcSubnetUpdate> = {
     name: subnet.name,
     description: subnet.description,
     customRouter: customRouterDataToForm(subnet.customRouterId),
diff --git a/test/e2e/vpcs.e2e.ts b/test/e2e/vpcs.e2e.ts
index 6f9f7f20d..f9ee5a207 100644
--- a/test/e2e/vpcs.e2e.ts
+++ b/test/e2e/vpcs.e2e.ts
@@ -63,6 +63,11 @@ test('can create and delete subnet', async ({ page }) => {
 
   await dialog.getByRole('textbox', { name: 'Name' }).fill('mock-subnet-2')
   await dialog.getByRole('textbox', { name: 'IPv4 block' }).fill('10.1.1.2/24')
+
+  // little hack to catch a bug where we weren't handling empty input here properly
+  await dialog.getByRole('textbox', { name: 'IPv6 block' }).fill('abc')
+  await dialog.getByRole('textbox', { name: 'IPv6 block' }).clear()
+
   await dialog.getByRole('button', { name: 'Create subnet' }).click()
 
   await expect(dialog).toBeHidden()