passing const pointer by reference

I am confused that why following code is not able to compile

int foo(const float* &a) {
    return 0;
}
int main() {
    float* a;
    foo(a);

    return 0;
}

Compiler give error as:

error: invalid initialization of reference of type 'const float*&' from expression of type 'float*'

but when I try to pass without by reference in foo, it is compiling fine.

I think it should show same behavior whether I pass by reference or not.

Thanks,