Debug Configuration Vs Release Configuration

I have these two namespaces in my class.

using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;

When build with debug configuration, everything works fine but in Release configuration I get an error:

Warning 18 Could not resolve this reference. Could not locate the assembly "DocumentFormat.OpenXml, Version=2.0.5022.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. IDDE.OpenOfficeXml

Any ideas?