Debugging Class libraries or DLL files through web application

Question

Hi,

I have a solution which consists of class library project and one web application. I created a dll from the class library project and add it as a reference to web application. I am really having difficulty in debugging. When i run default.aspx page it display error as "Value cannot be null.
Parameter name: format".

I am quite sure this error comes from DLL file. So could anyone help me to understand how to debug class libraries or DLL files together with web application side by side.