Can't add a const ubyte to a dynamic array of ubyte?

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import std.stdio;

ubyte[] extend(in uint[] arr)
{
        ubyte[] result;
        foreach (n; arr)
        {
                if (n < 10)
                {
                        result ~= n;
                        // source/app.d(10,11): Error: cannot 
append type const(uint) to type ubyte[]
                }
                else
                {
                        import std.conv : to;
                        foreach (digit; n.to!string)
                        {
                                result ~= digit.to!ubyte;
                        }
                }
        }
        return result;
}

unittest
{
        import std.algorithm : equal;

        assert(extend([1, 25, 70, 0]).equal([1, 2, 5, 7, 0, 0]));
}


How can I get around this? I want to ensure that the array is not 
mutated in the function in the signature too.