sfFTPLib exception thrown from .NET DLL

I have an application that calls a DLL that uses sfFTPLib. This DLL is called using System.Reflection but fails each time with an error of "System.Reflection.TargetInvocationException" and an inner error of
"Could not load file or assembly 'Interop.sfFTPLib, Version=3.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. An attempt was made to load a program with an incorrect format."

I am using C# with the latest version of sfFTPLib.dll. All files necessary for sfFTPLib are available to the DLL. Has anyone run across this before and have a solution?

Thanks.

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