Flow and function definitions

I'm trying to define a function signature in flow. I would expect the following code to throw an error but I'm not getting any error for this.

// function that applied a function to an array - just for numbers
const arrayMath = (func: () => number, arr: Array<number>):Array<Number> => arr.map(func);

// would expect this function to work OK with arrayMath
const square = (num: number): number => num * num;

// would expect this function to fail with arrayMath
const concatX = (str: string): string => `${str}X`;

// flow is not giving me an error here..
arrayMath(concatX, [1, 2, 3]);

via TimRich