Ternary operator doesn't work in Icarus Verilog

I have the following Verilog modules, which I'm simulating with Icarus Verilog. It seems that the ternary operator doesn't work, or has a bug:

main module (i2c_app_tb.v)

`timescale 1ns/100ps

module i2c_app_tb;

    wire t1;
    wire [7:0] t2;

    reg rst = 0;
    reg clk = 0;

    pullup(t1);

    pullupdown_t1 pt1(t1, t2, rst, clk);

    always #5 clk <= ~clk;

    initial begin
        $dumpfile("i2c_app_tb.vcd");
        $dumpvars(1, i2c_app_tb);

            rst <= 1;
        #20 rst <= 0;
        #300;
        $finish;
    end

    wire _i0_reg;
    assign _i0_reg = pt1.i0_reg;

endmodule

pullupdown_t1.v

module pullupdown_t1 (
    inout wire i0,
    inout wire [7:0] i1,
    input rst,

    input clk
);
    reg i0_reg;

    always @ (posedge clk or posedge rst) begin
        if (rst) begin
            i0_reg <= 0;
        end else begin
            i0_reg <= (i0_reg == 0 ? (1'bz) : 0);
        end
    end

    assign i0 = i0_reg;

endmodule

When I simulate the above, I get:

However, if I replace

i0_reg <= (i0_reg == 0 ? (1'bz) : 0);

by

if (i0_reg == 0) begin
    i0_reg <= 1'bz;
end else begin
    i0_reg <= 0;
end

(which is equivalent), I get:

Am I missing something? Is this a bug?