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.