Issue with template function

[Charles via Digitalmars-d-learn]

I'm trying to create a template function that can take in any 
type of array and convert it to a ubyte array. I'm not concerned 
with endianness at the moment, but I ran into a roadblock when 
trying to do this with strings. It already works with ints, 
chars, etc.

Here's the relevant test code:

     module byteReader;

     public import std.system : Endian;

     ubyte[] toBytes(T)(T[] arr)
     {
         if (arr == null)
         {
             return null;
         }

         ubyte[] result = new ubyte[arr.length];

         foreach (key, val; arr)
         {
             result[key] = cast(ubyte) val;    // This is line 16
         }

         return result;
     }

     string readString(ubyte[] buffer, uint offset, uint length)
     {
         assert( buffer.length >= offset + length );

         char[] chars = new char[length];
         foreach(key, val; buffer[offset .. offset + length])
         {
             chars[key] = cast(char) val;
         }

         return cast(string)chars;

     }

     void main() {
         import std.stdio;
         readString(toBytes!char(['t','e','s','t']),0,4).writeln;
         readString(toBytes!string("test"),0,4).writeln;    // 
This is line 39
     }

Here's the output:
     byteReader.d(16): Error: cannot cast val of type string to 
type ubyte
     byteReader.d(39): Error: template instance 
byteReader.toBytes!string error instantiating