If parity is enabled (see the .Settings property) then the com port will complete a parity check on each byte as is it received. Any byte failing that parity check will be removed from the data stream and replaced by the ParityReplace character. The MSComm32 default is the "?" questionmark character.
This property is a string so setting it to a string containing a number of characters would not throw an error but the property will ignore all but the first character