DUMP GETWA_NOT_ASSIGNED by ASSIGN FS dynamically

I got a DUMP GETWA_NOT_ASSIGNED, which is triggered by the ASSIGN statement. I once raised this problem. The code is as:

data: name3(21) value '(SAPLMBWL)IMSEG-ABLAD'.

assign (name3) to <f>.

here, IMSEG seems to be a interface of some FM. At last time, I was suggested to transfer the name3 to upper case. It works. But in some condition, it will also raise a DUMP. So I am thinking about there should be some other problem.

In the DUMP text, I found this in ERROR analysis:

You address a global function interface, although the

respective function module is not active - that is, is

not in the list of active calls. The list of active calls

can be taken from this short dump.

I checked the loaded program in debug mode, the program SAPLMBWL is not in the loaded list. But the program can run 2 times for the ASSIGN statement, At the first time, it works fine without any problem. But at the 2nd time, it raise the DUMP.

I think it has sth to do with the memory or objects in runtime. Is there any way to check (SAPLMBWL)IMSEG-ABLAD before the ASSIGN statement?

How to check the active calls list mentioned in the ERROR analysis and what can be done for it? Any check?

thanks for help.