Not able to get correct last name in certain pattern

[Jithu0106]

Not able to get correct last name in certain pattern

name = HumanName("Dr. Mark B. Cua, MD",constants=CONSTANTS)

If last name has more than 3 characters then it will predict last name correctly otherwise wrong

Result :
Prefix: Dr.
First Name: Mark
Middle Name:
Last Name: B.
Suffix: Cua, MD

Expected:
Prefix: Dr.
First Name: Mark
Middle Name: B.
Last Name: Cua
Suffix: MD