problems with swiggle use in oscillating wall

Dear all users.
I’m going to simulate a oscillating wall. I used following commands:
# variable A atom swiggle(0,3,12)
# velocity wall set v_A 0.0 0.0
But I saw this error: <<cannot use swiggle in variable formula between runs>>
How can I fix this problem?
Any help would be appreciated.

The velocity command does a one-time setting
of velocities to a value. You can’t use it

to set v to a time-dependent value, which is
what you are trying to do, hence the error.

You probably want the fix move command which
will set positions/velocities continuously during
a run.