Captive Portal - Certificate Issue in Mobile devices

Hi Everyone, Hope you can help me. I have FortiGate 100E, currently running FortiOS 5.6 So I've setup a Captive Portal in one of my interface. Everything seems working fine until when I tried to connect to the SSID with Captive Portal using Mobile device, it prompts me a certificate issue before I even redirected to the Captive Portal page. But I don't get this error when I'm connecting using Laptop, when I connect to the SSID it redirects me automatically to the Captive Portal without getting any certificate error. Any thoughts why I get this error in Mobile devices? Hope you can help me. Regards!