There is an alternative way to extract the master extended private key (xprv): in the Electrum console, simply type this:
wallet.get_master_private_key('x/', gui.password_dialog())
Or for the master private key of a wallet created with Electrum 1.x, it's:
wallet.get_seed(gui.password_dialog())
Warning
Don't ever type your password or a private key into the console. In other words, don't do this:
wallet.get_seed('my-password')
Everything you type into the console is temporarily stored inside your wallet file unencrypted to make the command history feature (accessed via the up-arrow) work.