How to assign a space like ' ' to a field symbol

Hi experts,

I want to assign a space to a field symbol, so just to make the field not to be null. I coded as below, but I keep getting dump. Am I missing anyting or assign a space to field symbol is not possible.

   FIELD-SYMBOLS: <fs1>.

  <fs1> = ' '.
  UNASSIGN <fs1>.

Thanks in advance.