Tooltip display crashing WPF application.

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C#

WPF

ToolTip

I am having one application. In this we are raising tooltip when mouse pointer comes on Image or Label and shows information.
I am using code as

ctrlName.Dispatcher.Invoke(new Action(delegate
{
   ToolTipService.SetToolTip(ctrlName, szToolTipMsg);
}));

This code is crashing app when called.
Can someone point out why this is happening.
Infact, this code was working earlier.

What I have tried:

After lots of trial I replaced above code with following and it start working...

ctrlName.Dispatcher.Invoke(new Action(delegate
{
    ToolTip tt = new ToolTip();
    tt.Content = szToolTipMsg;
    ctrlName.ToolTip = tt;
}));

I want to know what is the reason for this is working. While the ToolTipService.SetToolTip call is working in some other application.