I'm getting NAN out of nowhere

[Binarydepth via Digitalmars-d-learn]

This is my code :
import std.stdio : writeln, readf;
void main()     {
        int[3] nums;
        float prom;
        foreach(nem; 0..2)      {
                writeln("input a number : ");
                readf(" %d", &nums[nem]);
                prom+=nums[nem];
        }
        writeln(prom/3.0);
}
I get prompted two times for a number and I then get NAN out of 
nowhere.