How to add custom .dll in ssrs report

Hi,

I am working on SSRS report with project server 2013 and I want to add custom dll in  SSRS report.I have create dll first then add into GAC Assembly After that i have add this dll in report project in SSRS.when i have try to deploy and build report,I have facing some issue like...

.An error Occurred during local report processing.
Failed to load expression host assembly.
Details : Cannot Perform CAS Asserts in Security Transparent methods.

Please Suggest me as soon as possible.