Problem with #(delay) in assign statement

While trying to model combinational delay in systemverilog the following was observed.

The below code didn't work

assign #(50ps) b = a;

b was always x state.

But the below code works

parameter real del = 50ps;

assign #(del) b = a;

I feel this is a bug in irun.

File: systemverilog

irun argument -TIMESCALE 1s/1ps

Version irun: 15.20-s008