strictNullChecks still allows to assign undefined to the variable declared as number

TypeScript Version: 2.0.0

Code

// strictNullChecks is on var x: number[] = []; var y: number = x[0]; y.toString(); // Cannot read property 'toString' of undefined

Expected behavior:
error TS2322: Type 'undefined' is not assignable to type 'number'

Actual behavior:
after compile with command tsc --strictNullChecks
Runtime exception: Cannot read property 'toString' of undefined

I thought strictNullChecks a there to prevent developer from seeing such kind of exceptions in run time.

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