Extension method call does not compile, but static method call to same code does compile

There is library A calling library B using a C# extension method.

I got a weird error from the C# compiler:

The type 'System.Windows.Forms.Control' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Windows.Forms, Version=4.0.0.0

None of the libraries A or B are dependent on System.Windows.Forms.Control, nor has any dependency of A a dependency on System.Windows.Forms.Control. System.Windows.Forms.Control is only referenced from another project in the same solution.

The weird thing is that if I change the call syntax to static method, it compiles successfully.

//static method syntax works fine
var leads = SourceLeadConfigurationHelper.GetLeads(enLeadSystem);

//extension method syntax cause error
//error The type 'System.Windows.Forms.Control' is defined in an assembly that is not referenced. 
var leads = enLeadSystem.GetLeads();

The extension method looks like:

public static class SourceLeadConfigurationHelper
{      
    public static IList<ChannelID> GetLeads(this LeadSystem leadSystem);
    public static IList<ChannelID> GetLeads(this SourceLeadConfiguration slc);
    public static IList<ChannelID> GetLeads(LeadSystem leadSystem, bool throwException);
}

So you see there is no problem with detecting which extension to use. LeadSystem is an enum, SourceLeadConfiguration is a class.

I have Visual Studio 2013 update 5, .NET Framework 4.0.