Function Error: cannot be read at compile time

[jmh530 via Digitalmars-d-learn]

I'm trying to run the following code (to create an array of 
uniform random variables) on the latest version of rdmd (2.067.1).

import std.random;
auto uniform_array(int len, float a, float b) {
        Random gen;
        float output[len];
        foreach(ref float i; output)
        {
                i = uniform(a, b, gen);
        }
        return output;
}

void main() {
        auto val2 = uniform_array(10, 0.0f, 1.0f);
}

But I get the error
testing_some_stuff.d(5): Error: variable len cannot be read at 
compile time
Failed: ["dmd", "-v", "-o-", "testing_some_stuff.d", "-I."]

I'm not sure why this is an error. The code worked before I put 
it into a function. The function seems to work when I hardcode in 
a length. It's only having some variable length that's a problem. 
I'm not sure why.