MSProject.Application FileSaveAs giving null reference exception

hi,

I am using" Microsoft.Office.Interop.MSProject.Application" in MS office 2010 project proffesional.  but it is giving null reference exception.Actually i am trying to  publish my project .mpp to project server 2010 using FileSaveAs metnod.Below are my paramaters any thing elses i need to pass:

 projApp.Application.FileSaveAs(
                   "C:\Path.mpp",      // file name
                   Microsoft.Office.Interop.MSProject.PjFileFormat.pjMPP,  // format
                   Type.Missing,               // backup
                   Type.Missing,               // ReadOnly
                   true,              // TaskInformation
                   Type.Missing,               // Filtered
                   Type.Missing,                  // Table
                   Type.Missing,                  // UserID
                   Type.Missing,                  // DB Password
                   "",     // FormatID
                  Type.Missing,                  // Map
                   Type.Missing,                  // Password
                   Type.Missing,                  // Write Reserve Password
                   Type.Missing,               // Clear Baseline
                   Type.Missing,               // Clear Actuals
                  Type.Missing,               // Clear ResourceRates
                   Type.Missing);              // Clear FixedCosts

Thanks & Regards, Krishna