Symbolic simulation results are not as expected

[cccccc96]

I am learning the use of VOSS2, and I tried to write a very simple STE code for Memory testing.
For this Memory, write at time 1 and read at time 2.
But the result of symbolic simulation of output singal "io_dataOut" at moment 2 is X, I don't know where is my problem?

• Below is the verilog code

module mem(
  input         clock,
  input  [31:0] io_dataIn,
  output [31:0] io_dataOut,
  input  [4:0]  io_addr,
  input         csen_n,
  input         wren_n
);
 reg [31:0] mem [4:0];
 always @(posedge clock) begin
     if(wren_n && !csen_n)
         mem[io_addr] <= io_dataIn;
 end
 always @(posedge clock) begin
     if(!wren_n && !csen_n)
         io_dataOut <= mem[io_addr];
 end
endmodule

• Below is the fl code

load "ste.fl";
let p = verilog2pexlif F ("-I"^DIR^"") "mem" ["mem.v"] [];
let ckt = pexlif2fsm p;
let ant = 
    "clock" is_clock 2 and 
    "csen_n" is 0 for 2 cycles and 
    "wren_n" is 1 for 1 cycle followed_by //write at one cycle
                0 for 1 cycle            //read at second cycle
    and 
    "io_addr[4:0]" is 2 for 2 cycles    //write/read for addr-2 
    and 
    "io_dataIn[31:0]" is 4 for 2 cycles //write_data is 4   
;
let vis = STE_debug ckt;
let ste = STE "-e" vis [] ant [] [];
ste;

• Below is the waveform diagram of symbolic simulation