Could not find file 'bin\Antlr3.Runtime.pdb' Error when creating deployment package

I'm getting the following error when tried to create deployment package.

"Copying file bin\Antlr3.Runtime.pdb to obj\Release\AspnetCompileMerge\Source\bin\Antlr3.Runtime.pdb failed. Could not find file 'bin\Antlr3.Runtime.pdb'"

Can someone tell me what is happening and how to solve it