Nullable type not being applied when exporting
Solution 1:
The only cause of this I can think of would be that the strictNullChecks compiler option is off. This is also included by the "strict" compiler option
Try adding:
"strict": true,
Or:
"strictNullChecks": true,
to your tsconfig.json file under the "compilerOptions" section.
In strict mode, your code seems to work fine. See this sandbox.