PureMessage for Lotus Domino doesn`t detect viruses

We have Domino 8.5 + PM 4.5 on it. We receive letter with zip file with viruses. In PM log i see:

5:23:42.20    (5)     Executing Job ------------------------------------ DEFAULT - Virus Check all Mails#NT00000982(8000)        tm_job.cpp:1052         
05:23:42.24    (5)     - Check attachments                                                                                        wd_file.cpp:402         
05:23:42.24    (6)     - Check attachment : transact_store.zip                                                                    wd_file.cpp:93          
05:23:42.50    (6)     Sophos (scan engine): return code: 0 ok                                                                    wd_chck.cpp:510         
05:23:42.50    (6)     - Decompressing file 'transact_store.zip'                                                                  wd_file.cpp:188         
05:23:42.50    (6)     - Check attachment : <document contents>                                                                   wd_file.cpp:93          
05:23:42.52    (6)     Sophos (scan engine): return code: 0 ok                                                                    wd_chck.cpp:510         
05:23:42.52    (6)     - Decompressing file '<document contents>'                                                                 wd_file.cpp:188         
05:23:42.52    (5)     Job completed------------------------------------                                                          tm_job.cpp:1082         
05:23:42.52    (5)     Releasing mail for routing.                                                                                tm_job.cpp:1277         
05:23:42.53    (5)     done with document NT00EAD23A

But when i open this file from this letter, Sophos protection detects virus!