My python function returning NoneType object is not callable

#1 
def bill_of_materials(**kwargs):
    def production(qty):
        for key, value in kwargs.items():
            total = {key : value*qty}
    return total


f  = bill_of_materials (a = 2, b = 3, c = 4)
qty = float(input('enter qty : '))
usage = f(qty)

The code returning NoneType Error.

#2

Take a closer look at bill_of_materials function, happens in it and what it returns.