Invalid function assignment is not detected

TypeScript Version: 2.5.0

Code

const a: (d: string) => number = (d) => d.length;
const b: (d?: string) => number = a;
b();

Expected behavior:
When compiled with --strict, the assignment of b should cause a type error along the lines of:

string | undefined is not assignable to type string.

Actual behavior:
The code compiles with --strict, then throws an exception at runtime on the expression d.length.

Thank you. This is disappointing, but I understand the reasons.

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