Swift Package Manager - unexpected 'commands' value (expected map)

I am trying to build a swift system package on Ubuntu which uses Berkeley DB. It generated following errors

<unknown>:0: error: unexpected 'commands' value (expected map) 
<unknown>:0: error: unable to load build file

My modulemap file looks like the following

module BerkeleyDB [system] { 
     header "/usr/include/db.h" 
     link "libdb-5.3" 
     export * 
}

And Package.swift is as follows

// swift-tools-version:4.0 
// The swift-tools-version declares the minimum version of Swift required to build this package.  
import PackageDescription  
let package = Package( 
     name: "BerkeleyDB", dependencies: [ // Dependencies declare other packages that this package depends on. 
                                        // .package(url: /* package url */, from: "1.0.0"), ] 
)