I get the following error message when trying to add a placeholder to a layout in Sitecore 6.5:

"Empty strings are not allowed.<br>Parameter name: xml"

Any idea what could be the problem? The following stack trace appears in the log:

```
1124 10:20:56 ERROR Application error.
Exception: System.Web.HttpUnhandledException
Message: Exception of type 'System.Web.HttpUnhandledException' was thrown.
Source: System.Web
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.sitecore_shell_default_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\59ed33b7\d6112557\App_Web_edgk4py3.1.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Nested Exception

Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at Sitecore.Reflection.ReflectionUtil.CallMethod(Type type, Object obj, String methodName, Boolean includeNonPublic, Boolean includeInherited, Object[] parameters)
   at Sitecore.Reflection.ReflectionUtil.CallMethod(Object obj, String methodName, Boolean includeNonPublic, Boolean includeInherited, Object[] parameters)
   at Sitecore.Web.UI.HtmlControls.Component.HandleMessage(Message message)
   at Sitecore.Shell.Framework.Commands.ClientEventCommand.SendEventMessage()
   at Sitecore.Shell.Framework.Commands.ClientEventCommand.Execute(CommandContext context)
   at Sitecore.Web.UI.Sheer.ClientPage.RaiseEvent()
   at Sitecore.Web.UI.Sheer.ClientPage.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Nested Exception

Exception: System.ArgumentException
Message: Empty strings are not allowed.
Parameter name: xml
Source: Sitecore.Kernel
   at Sitecore.Diagnostics.Assert.ArgumentNotNullOrEmpty(String argument, String argumentName)
   at Sitecore.Layouts.LayoutDefinition.Parse(String xml)
   at Sitecore.Shell.Applications.Layouts.DeviceEditor.DeviceEditorForm.OnOK(Object sender, EventArgs args)

```