openapi-ts [Cannot read properties of undefined (reading 'charAt')]

openapi-ts.config.ts

import { defineConfig } from "@hey-api/openapi-ts"

export default defineConfig({
  client: "legacy/axios",
  input: "./openapi.json",
  output: "./src/client",
  // exportSchemas: true,
  plugins: [
    {
      name: "@hey-api/sdk",
      // NOTE: this doesn't allow tree-shaking
      asClass: true,
      operationId: true,
      methodNameBuilder: (operation) => {
        // @ts-expect-error
        let name: string = operation.name
        // @ts-expect-error
        const service: string = operation.service

        if (service && name.toLowerCase().startsWith(service.toLowerCase())) {
          name = name.slice(service.length)
        }

        return name.charAt(0).toLowerCase() + name.slice(1)
      },
    },
  ],
})

openapi.json

{
  "openapi": "3.0.0",
  "info": {
    "title": "Test API",
    "version": "1.0.0"
  },
  "paths": {
    "/test": {
      "get": {
        "summary": "Test endpoint",
        "operationId": "testEndpoint",
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "message": {
                      "type": "string"
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

package.json

{
  "name": "test-reproduce",
  "version": "1.0.0",
  "scripts": {
    "generate-client": "openapi-ts"
  },
  "devDependencies": {
    "@hey-api/openapi-ts": "^0.90.10"
  }
}