I have a DLL library that uses a configuration file to get a value which can be set at the client.
The configuration file contains:

 <appSettings>
    <add key="CertificateSerialNumber" value="SerialNumber"/>
  </appSettings>

In the code of the library I try to get the serial number:

SerialNumber = ConfigurationSettings.AppSettings["CertificateSerialNumber"];

Which works fine in debug, but when executing the code at the client, the subsequent error occurs:

Value cannot be Null

So I assume it cannot find the parameter I am looking for. The config file is in the same folder as the dll, so what am I missing?