prefix of optional s does not throw error when capitalised

[dnardnar]

Expected: Error for invalid format of S/.
Actual: S/ Math is added to the address.
Suggestion/ Why it is problematic: It is possible for a typo in the prefix
Steps to reproduce:
add_student n/John Doe p/91234567 e/ johndoe@gmail.com a/ 311, Clementi Ave 2, #2-25 S/Math
Screenshot: