TypeError: Cannot read property 'toLowerCase' of undefined

[timocov]

TypeScript Version: 3.3.0-dev.20190126, 3.2.4

Search Terms: toLowerCase, TypeError, extendable configs

Code

tsconfig.json

{
	"compilerOptions": {
		"composite": true
	},
	"references": [
		{ "path": "./tsconfig.first.json" },
		{ "path": "./tsconfig.second.json" }
	]
}

tsconfig.first.json

{
	"extends": "./foobar.json",
	"compilerOptions": {
		"composite": true
	}
}

tsconfig.second.json

{
	"extends": "./foobar.json",
	"compilerOptions": {
		"composite": true
	}
}

tsc -b tsconfig.json

Expected behavior:

error TS5058: The specified path does not exist: 'foobar.json'.

Actual behavior:

TypeError: Cannot read property 'toLowerCase' of undefined

The reason of that error is somewhere here: https://github.com/Microsoft/TypeScript/blob/fa74cef81e488c8e5bf790020620f0226118cb58/src/compiler/program.ts#L225-L244

I don't know how it is even compiled, because readFileCache is map for string | false, so readFileCache.get() mush return string | false | undefined and after checking if (value !== undefined) in host.readFile function type of the value should be string | false, what is not assigned to string | undefined (according scriptNullChecks flag enabled).

UPD: setReadFileCache returns any (due disabled strictBindCallApply compiler option), that's why there is no error.