Reflection - trouble with loading dependent assembly

I am trying to execute below code to see if assembly was built in Debug or Release mode.

Assembly assemb = Assembly.LoadFile(fileName);
bool isDebug = false;
foreach (object att in assemb.GetCustomAttributes(false))
    if (att is DebuggableAttribute)
        isDebug = ((DebuggableAttribute)att).IsJITTrackingEnabled;
Console.WriteLine("Assembly is {0}.", isDebug ? "debug" : "release");

I am able to load assembly (Product.dll) without any issue. But when I am trying to execute GEtCustomAttributes(false) method I am getting below exception message.

Could not load file or assembly 'log4net, Version=1.2.11.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.

Don't know why this is looking for dependent assembly. Is it because of the way Product.dll was build ( like optimization or something ). I don't have access to source code of Product.dll so not sure how I can file its mode ( Debug or Release )