Invalid URL when using Mailgun provider

[notflip]

I'm getting the following error (on localhost), when using the Mailgun provider

My .env value for nextauth is set NEXTAUTH_URL=http://localhost:3000

[auth][error] TypeError: Invalid URL
    at new URL (node:internal/url:775:36)
    at Object.redirect (webpack-internal:///(rsc)/./node_modules/.pnpm/@auth+core@0.35.3_nodemailer@6.9.14/node_modules/@auth/core/lib/init.js:29:18)
    at sendToken (webpack-internal:///(rsc)/./node_modules/.pnpm/@auth+core@0.35.3_nodemailer@6.9.14/node_modules/@auth/core/lib/actions/signin/send-token.js:42:39)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Module.signIn (webpack-internal:///(rsc)/./node_modules/.pnpm/@auth+core@0.35.3_nodemailer@6.9.14/node_modules/@auth/core/lib/actions/signin/index.js:22:30)
    at async AuthInternal (webpack-internal:///(rsc)/./node_modules/.pnpm/@auth+core@0.35.3_nodemailer@6.9.14/node_modules/@auth/core/lib/index.js:72:24)
    at async Auth (webpack-internal:///(rsc)/./node_modules/.pnpm/@auth+core@0.35.3_nodemailer@6.9.14/node_modules/@auth/core/index.js:127:34)
    at async signIn (webpack-internal:///(rsc)/./node_modules/.pnpm/next-auth@5.0.0-beta.22_next@14.2.6_react-dom@18.3.1_react@18.3.1__react@18.3.1__nodemailer@6.9.14_react@18.3.1/node_modules/next-auth/lib/actions.js:53:17)
    at async $$ACTION_0 (webpack-internal:///(rsc)/./src/app/auth/login/page.tsx:65:5)
    at async /Users/miguelstevens/Code/@playground/nextjs-shlink/node_modules/.pnpm/next@14.2.6_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/next/dist/compiled/next-server/app-page.runtime.dev.js:39:418
    at async rw (/Users/miguelstevens/Code/@playground/nextjs-shlink/node_modules/.pnpm/next@14.2.6_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/next/dist/compiled/next-server/app-page.runtime.dev.js:38:7978)
    at async r6 (/Users/miguelstevens/Code/@playground/nextjs-shlink/node_modules/.pnpm/next@14.2.6_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/next/dist/compiled/next-server/app-page.runtime.dev.js:41:1256)
    at async doRender (/Users/miguelstevens/Code/@playground/nextjs-shlink/node_modules/.pnpm/next@14.2.6_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/next/dist/server/base-server.js:1438:30)

This is my code

page.tsx

<div className="w-full h-screen flex items-center justify-center px-4">
        <SignIn
          action={async (formData: FormData) => {
            "use server";
            await signIn("mailgun", formData);
          }}
        />
      </div>

sign-in.tsx

<form action={action}>
            <input
            name="email"
            id="email"
            type="email"
            placeholder="jane@gmail.com"
            required
          />
        <SubmitButton />
    </form>

Does anyone know what might be the cause of this?