newbie question about variables in slices..

[Kai via Digitalmars-d-learn]

Hi I am trying to iterate over a mmfile (ubyte[]) and convert it 
to uint

void main(){
	MmFile inn = new MmFile("mmData.dat");
	ubyte[] arr = cast(ubyte[])inn[];
	for(ulong index = 0; index<arr.length; index+=4){
		ulong stop = index+4;	
		uint num  = littleEndianToNative!uint(arr[index..stop]);
	}
if i try to compile this i get the following error:
Error: template std.bitmanip.littleEndianToNative cannot deduce 
function from argument types !(uint)(ubyte[])

but if change the last line to:
uint num  = littleEndianToNative!uint(arr[30..34]);

then it compiles and runs...

Am I doing something wrong with my variables "index" and "stop"?
cheers
Kai T