avoid compiler warning by raising InvalidOperationException with proper message
avoid compiler warning by raising InvalidOperationException with proper message

ploeh/Numsense,ErikSchierboom/Numsense,ploeh/Numsense,simegeorge/Numsense,albertodall/Numsense,albertodall/Numsense,simegeorge/Numsense,ErikSchierboom/Numsense