Netlifyでデプロイするとエラーが出てしまう

Nuxt.JSで作成したブログをNetlifyでデプロイすると、
1「TypeError [ERR_HTTP_INVALID_HEADER_VALUE]: Invalid value "undefined" for header "X-API-KEY"」といったエラーがでてしまいます。原因と対策をご存知の方はいらっしゃいませんか？
2X-API-KEY　は、.envファイルにて、
3BASE_URL=http://localhost:3000
4API_KEY=○○○○○○○○○○○
5API_URL=○○○○○○○○○○○
6
7nuxt.config.jsにて、
8const { API_KEY,API_UR } = process.env
9
10と設定しています。
11そして、/blog/index.vueにて、
12import axios from 'axios'
13export default {
14  async asyncData({ $config }) {
15    const { data } = await axios.get(`${$config.apiUrl}/blog`, {
16      headers: { 'X-API-KEY': $config.apiKey },
17    })
18    return {
19      items: data.contents,
20    }
21  },
22としてます。npm run devではエラーもでないのですが、
23Netlifyでデプロイすると以下のようなログとなります。
24
25どうしたらいいでしょうか？
26
279:08:10 PM: $ npm run generate
289:08:10 PM: > ESSENTIAL@1.0.0 generate /opt/build/repo
299:08:10 PM: > nuxt generate
309:09:05 PM: [error]  /blog
319:09:05 PM: 
329:09:05 PM: TypeError [ERR_HTTP_INVALID_HEADER_VALUE]: Invalid value "undefined" for header "X-API-KEY"
339:09:05 PM:     at ClientRequest.setHeader (_http_outgoing.js:521:3)
349:09:05 PM:     at new ClientRequest (_http_client.js:241:14)
359:09:05 PM:     at Object.request (http.js:46:10)
369:09:05 PM:     at RedirectableRequest._performRequest (/opt/build/repo/node_modules/follow-redirects/index.js:265:24)
379:09:05 PM:     at new RedirectableRequest (/opt/build/repo/node_modules/follow-redirects/index.js:61:8)
389:09:05 PM:     at Object.request (/opt/build/repo/node_modules/follow-redirects/index.js:456:14)
399:09:05 PM:     at dispatchHttpRequest (/opt/build/repo/node_modules/axios/lib/adapters/http.js:195:25)
409:09:05 PM:     at new Promise (<anonymous>)
419:09:05 PM:     at httpAdapter (/opt/build/repo/node_modules/axios/lib/adapters/http.js:46:10)
429:09:05 PM:     at dispatchRequest (/opt/build/repo/node_modules/axios/lib/core/dispatchRequest.js:52:10)
439:09:05 PM:     at runNextTicks (internal/process/task_queues.js:62:5)
449:09:05 PM:     at listOnTimeout (internal/timers.js:518:9)
459:09:05 PM:     at processTimers (internal/timers.js:492:7)
469:09:05 PM:     at async asyncData (pages/blog/index.vue:33:0)
479:09:05 PM: [error] Invalid value "undefined" for header "X-API-KEY"
489:09:05 PM:   at ClientRequest.setHeader (_http_outgoing.js:521:3)
499:09:05 PM:   at new ClientRequest (_http_client.js:241:14)
509:09:05 PM:   at Object.request (http.js:46:10)
519:09:05 PM:   at RedirectableRequest._performRequest (node_modules/follow-redirects/index.js:265:24)
529:09:05 PM:   at new RedirectableRequest (node_modules/follow-redirects/index.js:61:8)
539:09:05 PM:   at Object.request (node_modules/follow-redirects/index.js:456:14)
549:09:05 PM:   at dispatchHttpRequest (node_modules/axios/lib/adapters/http.js:195:25)
559:09:05 PM:   at new Promise (<anonymous>)
569:09:05 PM:   at httpAdapter (node_modules/axios/lib/adapters/http.js:46:10)
579:09:05 PM:   at dispatchRequest (node_modules/axios/lib/core/dispatchRequest.js:52:10)
589:09:05 PM:   at runNextTicks (internal/process/task_queues.js:62:5)
599:09:05 PM:   at listOnTimeout (internal/timers.js:518:9)
609:09:05 PM:   at processTimers (internal/timers.js:492:7)
619:09:05 PM:   at async asyncData (pages/blog/index.vue:33:0)
629:09:07 PM: 
639:09:07 PM: (build.command completed in 57.1s)