TypeError: selection.isMergeable is not a function at VeUiMergeCellsContextItem.ve.ui.MergeCellsContextItem.setup (http://dev.wiki.local.wmftest.net:8080/w/extensions/VisualEditor/lib/ve/src/ui/contextitems/ve.ui.MergeCellsContextItem.js?82ffd:56:27) at VeUiDesktopContext.ve.ui.Context.setupMenuItems (http://dev.wiki.local.wmftest.net:8080/w/extensions/VisualEditor/lib/ve/src/ui/ve.ui.Context.js?b3717:160:14) at VeUiDesktopContext.ve.ui.LinearContext.afterContextChange (http://dev.wiki.local.wmftest.net:8080/w/extensions/VisualEditor/lib/ve/src/ui/contexts/ve.ui.LinearContext.js?4f225:118:10) at VeUiDesktopContext.ve.ui.DesktopContext.afterContextChange (http://dev.wiki.local.wmftest.net:8080/w/extensions/VisualEditor/lib/ve/src/ui/contexts/ve.ui.DesktopContext.js?07a30:74:58)
i.e. isMergeable = selection.isMergeable();
...and then it changes the context item to look like this:

