User-defined literal string: compile-time length check

I have a user-defined literal operator that only makes sense for strings of a specific length, like this:

constexpr uint16_t operator "" _int(const char* s, std::size_t len)
{
    return len == 2 ? s[0] | (s[1] << 8) : throw;
}

This works:

"AB"_int // equals 16961

But this also compiles, and I don't want it to:

"ABC"_int // throws at runtime

I tried static_assert(len == 2), but it isn't allowed in a constexpr function.

How can I make "ABC"_int cause an error at compile time?