Alter Procedure gave "Quest.Toad.SqlServer.Trl.TrlHasEmptyResultSetException". (4280070)

In the Object Explorer, clicking on the "Alter Stored Procedure" icon (or right-click on the stored procedure and choosing "Alter Stored Procedure") causes the following exception:

Quest.Toad.SqlServer.Trl.TrlHasEmptyResultSetException
FastTable.Rows.Count = 0, FastTable.Columns.Count = 18 for: sqlserver://<server>/STOREDPROC/<database>/<procedure>//dbo/PROPERTIES
Stack Trace:
   at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous)
   at System.Windows.Forms.Control.Invoke(Delegate method, Object[] args)
   at System.Windows.Forms.WindowsFormsSynchronizationContext.Send(SendOrPostCallback d, Object state)
   at Quest.Toad.Common.Services.SynchronizationContextSchedulerService.Schedule(WaitCallback callback, Guid workflowInstanceId)
   at System.Workflow.Runtime.WorkflowExecutor.RequestHostingService()
   at System.Workflow.Runtime.ScheduleWork.Dispose()
   at System.Workflow.Runtime.WorkflowExecutor.EnqueueItem(IComparable queueName, Object item, IPendingWork pendingWork, Object workItem)
   at System.Workflow.Runtime.WorkflowInstance.EnqueueItem(IComparable queueName, Object item, IPendingWork pendingWork, Object workItem)
   at Quest.Toad.Common.Activities.LoadTrlDataActivity.InvokeHolder.OnTrl_Updated(Object oUnused, EventArgs eventArgs)
   at Quest.Toad.Cache.LiveCache.UpdatedProxy(Object sender, EventHandler handlers)