Firebase-tools: Cannot deploy functions; The engine "node" is incompatible with this module. Expected version ">=10"

[joelclimbsthings]

Using firebase-tools 6.12.0.

I haven't changed anything relevant, and hardly anything at all, but I'm suddenly getting the following error when attempting to deploy any of my Node 8 functions to firebase:

Build failed: {"error": {"canonicalCode": "INVALID_ARGUMENT", "errorMessage": "`yarn_install` had stderr output:\nerror [email protected]: The engine \"node\" is incompatible with this module. Expected version \">=10\".\nerror Found incompatible module\n\nerror: `yarn_install` returned code: 1", "errorType": "InternalError", "errorId": "179DED19"}}

I've tried updating core firebase packages but it hasn't bad a difference. I have the following specifying node 8 in my package.json:

"engines": { "node": "8" }

Help?

[joelclimbsthings]

No idea why, but deleting yarn.lock from the uploaded directory first fixed the issue.