wpf contracting stringformat doesn't work tooltips

the following formula rudimentary contracting binds calm textblock named mytextblock textbox's calm tooltip ability controlling accurate same contracting notation:

<stackpanel>
    <textblock x:name="mytextblock">foo bar</textblock>
    <textbox    text="{binding elementname=mytextblock, path=text, stringformat='it is: \{0\}'}"
             tooltip="{binding elementname=mytextblock, path=text, stringformat='it is: \{0\}'}" />
</stackpanel>

the contracting also uses operative glorious above calm ability nonetheless seems damaged tooltip. approaching outcome "it is: foo bar" nonetheless float over textbox, tooltip shows wholly contracting value, twine formatted value. any ideas?