Method not found: 'Boolean EPiServer.Core.PageLanguageSettingsTree.IsLanguageAllowedForCreation(EPiServer.Core.PageReference, System.String)'.

MissingMethodException: Method not found: 'Boolean EPiServer.Core.PageLanguageSettingsTree.IsLanguageAllowedForCreation(EPiServer.Core.PageReference, System.String)'.]
   EPiServer.UI.Edit.PageExplorer.PageTreeView_ItemDataBound(Object sender, PageTreeViewEventArgs e) +0
   EPiServer.UI.WebControls.PageTreeView.CreateTemplatedControls(ITemplate template, PageTreeNode treeNode, Int32 position, Boolean dataBinding) +223
   EPiServer.UI.WebControls.PageTreeView.CreateChildControlsFromItems(Boolean dataBinding) +155
   EPiServer.UI.WebControls.PageTreeView.PerformDataBinding() +205
   System.Web.UI.WebControls.HierarchicalDataBoundControl.PerformSelect() +114
   EPiServer.UI.Edit.PageExplorer.OnLoad(EventArgs e) +623
   System.Web.UI.Control.LoadRecursive() +66
   System.Web.UI.Control.LoadRecursive() +191
   System.Web.UI.Control.LoadRecursive() +191
   System.Web.UI.Control.LoadRecursive() +191
   System.Web.UI.Control.LoadRecursive() +191
   System.Web.UI.Control.LoadRecursive() +191

...

Just popped up out of nowhere. The error occurs in EPiServer is trying to render the PageTree. Any ideas?