Problem with IndexError

I’m having a problem with IndexError in CP2.2.0. My pipeline stops in about one every 30 images with the error:
Worker 1: Fri Jan 25 16:52:47 2019: Image # 33, module CalculateMath # 20: 0.00 secs
Worker 1: Error detected during run of module FilterObjects#21
Worker 1: Traceback (most recent call last):
Worker 1: File “cellprofiler\pipeline.pyc”, line 1956, in run_image_set
Worker 1: File “cellprofiler\pipeline.pyc”, line 2067, in run_module
Worker 1: File “cellprofiler\modules\filterobjects.pyc”, line 469, in run
Worker 1: IndexError: index 39 is out of bounds for axis 1 with size 39

FilterObjects takes a value from CalculateMath.
Any ideas what this error means and I should be looking for in order to avoid this error?
Thanks very much