Verilog-A transition filter warning from spectre

I'm using transition filter to create a verilog A model for a sample and hold with finite rise and fall time, unlike the ahdlLib sample and hold which has very sharp rise and fall time. My code is as below

module sah_ideal_risefall(vin, vout, vclk);
input vin, vclk;
output vout;
electrical vin, vout, vclk;
parameter real vtrans_clk = 2.5;
parameter real risefalltime = 15p;
real vout_val;

    analog begin
        @(initial_step) begin
            vout_val = 0;
        end
        @(cross(V(vclk) - vtrans_clk, +1)) begin
            vout_val = V(vin);
        end
        V(vout) <+ transition(vout_val,0,risefalltime);
    end
endmodul

During simulation spectre is giving a warning as,

line 22: Transition filter contains a continuous signal in the expression. Since a transition filter can only be applied to piecewise constant signals, redefine the expression to ensure that the transition filter applies only to a piecewise constant signal.

Since I was updating the variable at the rising edge of the clock, I assumed the variable will be piecewise linear. If so this warning shouldn't be appearing. Does anyone have any idea why this warning is showing up?

Thank you in advance