Oracle 12c Database Connection Problem

Hello, i have already set up an Oracle Database but I am facing a connectivity problem in the below manner:

sqlplus CISADM/CISADM@<IP>:1521/PSRM

sqlplus CISADM@PSRM

With the above two options I successfully connect to the database. Although, when I try to connect as: sqlplus CISADM@<IP>:1521/PSRM I get the following error :

ORA-12504: TNS:listener was not given the SERVICE_NAME in CONNECT_DATA

I can perform ping either with IP, or hostname, tnsping with IP and hostname successfully.

My tnsnames.ora file:

PSRM =

  (DESCRIPTION =

    (ADDRESS_LIST =

      (ADDRESS = (PROTOCOL = TCP)(HOST = <IP>)(PORT = 1521))

    )

    (CONNECT_DATA =

      (SERVER = DEDICATED)