External Call Thru UTL_HTTP("https:///servlet?parameter)

Hi All,
I have a requirement to call externalsystem from pl/sql. and externalURL needs just one parameter . When i tried i got below error

RA-29273: HTTP request failed
ORA-06512: at "SYS.UTL_HTTP", line 1527
ORA-29261: bad argument
ORA-06512: at "GLOGOWNER.SHOW_HTML_FROM_URL", line 22
ORA-29273: HTTP request failed
ORA-06512: at "SYS.UTL_HTTP", line 1130
ORA-29024: Certificate validation failure
ORA-06512: at line 1
29273. 00000 - "HTTP request failed"
*Cause: The UTL_HTTP package failed to execute the HTTP request.
*Action: Use get_detailed_sqlerrm to check the detailed error message.
Fix the error and retry the HTTP request.


I am using OTM 6.3.
please help me to resolve this issue.