Game crashes when animation does not properly animation interval

[burtlo]

When I mistakenly defined an animation with a duration and not an interval the game crashes. A warning should be specified if an interval is not found and it should default to 1.tick

animate :speech_bubble, to: { alpha: 255 }, duration: 16.ticks

/Users/burtlo/.rvm/gems/ruby-1.9.3-p392@gaming/gems/chipmunk-5.3.4.5/lib/chipmunk.rb:6: Use RbConfig instead of obsolete and deprecated Config.
WARNING: Nokogiri was built against LibXML version 2.9.0, but has dynamically loaded 2.7.8
/Volumes/Amazon/metro/metro/lib/metro/animation/on_update_operation.rb:76:in `>=': comparison of Fixnum with nil failed (ArgumentError)
    from /Volumes/Amazon/metro/metro/lib/metro/animation/on_update_operation.rb:76:in `update_completed?'
    from /Volumes/Amazon/metro/metro/lib/metro/animation/on_update_operation.rb:42:in `update'
    from /Volumes/Amazon/metro/metro/lib/metro/scene.rb:376:in `block in base_update'
    from /Volumes/Amazon/metro/metro/lib/metro/scene.rb:376:in `each'
    from /Volumes/Amazon/metro/metro/lib/metro/scene.rb:376:in `base_update'
    from /Volumes/Amazon/metro/metro/lib/metro/window.rb:41:in `update'
    from /Volumes/Amazon/metro/metro/lib/metro/game.rb:18:in `start!'
    from /Volumes/Amazon/metro/metro/lib/metro.rb:108:in `start_game'
    from /Volumes/Amazon/metro/metro/lib/metro.rb:100:in `run'
    from ../metro/bin/metro:15:in `<main>'