"SSL certificate untrusted" ?

"SSL certificate untrusted" ?

I'm trying to create a new SSL certificate monitor.  I'm pointing it at a client's website.  It has a valid SSL cert that will expire notAfter=Aug 19 23:59:59 2017 GMT.  I can talk to the website using `openssl s_client` and Google Chrome and Mozilla Firefox.  But when I try to create an SSL certificate monitor, I get an error message telling me I didn't configure the monitor correctly because "SSL certificate untrusted".

openssl s_client -servername ${HOSTNAME} -tlsextdebug -msg -connect ${HOSTNAME}:443 </dev/null | openssl x509  -noout -subject -issuer -dates

subject= /CN=[hostname omitted]
issuer= /C=US/O=GeoTrust Inc./OU=Domain Validated SSL/CN=GeoTrust DV SSL CA - G3
notBefore=Jun 21 00:00:00 2016 GMT
notAfter=Aug 19 23:59:59 2017 GMT

Why would site24x7 balking?  Is there a blacklist I should be aware of?