Summary

#10959 added url = url || '' to buildURL to stop a TypeError when params were passed with an undefined url. That guard also changed what a nullish url means. axios.get(null, { params }) used to blow up before any request went out; since 1.18.1 it builds ?foo=bar, which the browser resolves against the current page, so the app silently fires a request at itself.

The empty string is a legitimate relative url and should keep working. null and undefined are not, and they should not quietly become one. This restores the fail-fast behavior without bringing back the raw TypeError: a non-string url with params now throws an AxiosError carrying ERR_INVALID_URL, the same code buildFullPath already uses for unusable urls. Values such as 0, false and a URL instance previously reached url.indexOf and died with a bare TypeError, so they are covered by the same check. A url that comes from baseURL alone is unaffected, since combineURLs has already turned it into a string by then.

Scope is deliberately limited to the params path, which is the part #10959 changed. axios.get(null) without params returns early at if (!params) and behaves exactly as it did before 1.18.1.

Linked issue

Closes #11160

Changes

• lib/helpers/buildURL.js: throw AxiosError / ERR_INVALID_URL for a non-string url instead of coercing it to ''.
• tests/unit/helpers/buildURL.test.js: cover the rejection and keep the empty-string case explicit.
• tests/unit/helpers/resolveConfig.test.js: regression test for the reported path, a null url plus params no longer resolves to the current page.
• PRE_RELEASE_CHANGELOG.md: note the fix.

Testing

• npx eslint lib/helpers/buildURL.js
• npm run test:vitest:unit — 58 files, 1,064 tests passed

Checklist

• Tests added or updated
• No public API type change
• Behavior change is intentional and restores pre-1.18.1 semantics for nullish urls