Associative Array Problem

[Wolftousen Frozenwind]

I'm trying to have an associative array with the key value of type ushort.
\
struct struct_name
{
     int variable_name;
}

ushort[some_number] keys;

//fill keys

//declare my associative array
struct_name[ushort] data;

//in a loop, filling data
for(x = 0; x < keys.length; x++)
     data[keys[x]].variable_name = some_int;

I get an array out of bounds error when running this.  Any one help?