IDEAで.html.erbのファイルが編集できない。

Ruby及びRailsのサポートが充実してきているIDEAを使ってRoRの開発を始めたけど、何故か拡張子が「.html.erb」というファイルを編集しようとすると、IDEAのエディタが固まってしまい、何も入力を受け付けなくなる。

IDEAのログを見ると、

java.lang.AssertionError
	at com.intellij.extapi.psi.PsiFileBase.<init>(PsiFileBase.java:11)
	at com.intellij.freemarker.psi.files.FtlFile.<init>(FtlFile.java:60)
	at com.intellij.freemarker.FtlParserDefinition.createFile(FtlParserDefinition.java:71)
	at org.jetbrains.plugins.ruby.erb.ERbFileViewProvider.createFile(ERbFileViewProvider.java:84)
	at com.intellij.psi.MultiplePsiFilesPerDocumentFileViewProvider.getPsiInner(MultiplePsiFilesPerDocumentFileViewProvider.java:62)
	at com.intellij.psi.SingleRootFileViewProvider.getPsi(SingleRootFileViewProvider.java:41)
	at com.intellij.psi.impl.source.PsiFileImpl.getPsiRoots(PsiFileImpl.java:313)
	at com.intellij.psi.util.PsiUtilBase.getRootIndex(PsiUtilBase.java:272)
	at com.intellij.codeInsight.daemon.impl.analysis.HighlightingSettingsPerFile.getHighlightingSettingForRoot(HighlightingSettingsPerFile.java:66)
	at com.intellij.codeInsight.daemon.impl.analysis.HighlightLevelUtil.shouldHighlight(HighlightLevelUtil.java:13)
	at com.intellij.codeInsight.daemon.impl.LineMarkersPass.collectInformationWithProgress(LineMarkersPass.java:11)
	at com.intellij.codeInsight.daemon.impl.ProgressableTextEditorHighlightingPass.doCollectInformation(ProgressableTextEditorHighlightingPass.java:15)
	at com.intellij.codeHighlighting.TextEditorHighlightingPass.collectInformation(TextEditorHighlightingPass.java:13)
	at com.intellij.codeInsight.daemon.impl.PassExecutorService$ScheduledPass$1$1.run(PassExecutorService.java:2)
	at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:343)
	at com.intellij.codeInsight.daemon.impl.PassExecutorService$ScheduledPass$1.run(PassExecutorService.java:2)
	at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:13)
	at com.intellij.codeInsight.daemon.impl.PassExecutorService$ScheduledPass.run(PassExecutorService.java:21)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
	at java.util.concurrent.FutureTask.run(FutureTask.java:138)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:619)