Can't fix error in Verilog

I'm trying to make random 4-bit number generator not repeating previous 
output.
 Can't find any reason why there is an error on code
  "IndexNumber <= zero;"

these are some part of code:

module RandomNumberGen(

  output reg [3:0] RandomIndex

    );

reg [3:0] IndexNumber [0:9];
reg [9:0] record;
reg seed;
integer i;


  parameter   zero   = 4'b0000;
  parameter   one    = 4'b0001;
  parameter   two    = 4'b0010;
  parameter   three  = 4'b0011;
  parameter   four   = 4'b0100;
  parameter   five   = 4'b0101;
  parameter   six    = 4'b0110;
  parameter   seven  = 4'b0111;
  parameter   eight  = 4'b1000;
  parameter   nine   = 4'b1001;

  IndexNumber [0] <= zero; /* makes error!! */


endmodule