'const' initializer in an ambient context must be a string or numeric literal

I have a namespace in typescript that looks like this :

export declare namespace MyNamespace {
    const myFunc = (a:number) => 3;
}

My compiler complains about the function that :

TS1254: A 'const' initializer in an ambient context must be a string or numeric literal or literal enum reference

What is this error trying to tell me? I cannot have a function as a const within a namespace?