applyImpulse on Merged meshes cause null exception

After merging 2 meshes trying to call applyImpulse results in a null exception something like this:

Uncaught TypeError: Cannot read property 'applyImpulse' of undefined
    at CannonJSPlugin.applyImpulse (es6.js:81725)
    at PhysicsImpostor.applyImpulse (es6.js:80780)

Same happens with Oimo.

Replicated here: http://playground.babylonjs.com/#UGLFTI#1