variables in jbpm4_hist_var have no execution id

I use this code to write variables to jbpm4_hist_var.
"executionService.createVariable(task.getExecutionId(),dataMap,true);"

when I check database,I found the column 'executionid' always null.

I don't know how to write the example testcase to discribe this condition,but I will do my best to add more information about this issue if you need.