构建引用dll的VB.NET 2008 express项目的问题

I have written a program in VB.NET 2008 Express that references a .NET managed dll. I simply added a reference to the dll to the project. I then 'Imports' the dll at the beginning of the program and then can use it. Everything works fine when I am running the program in debug mode (and it uses the dll as intended), but as soon as I go to build the project I get an errors that it "Could not reslove this reference" and "Namespace of type specifier in the Imports 'ModbusTCP' doesn't contain any public member or cannot be found."I have written a program in VB.NET 2008 Express