[Zope] ZPT : prevent unauthorized error

[kent sin]

<img src="icon" tal:attributes="src item1/icon; alt
item1/title_or_id">

Produce the following

Error Type: Undefined
Error Value: "You are not authorized to access icon."
not found in "item1/icon"

How could I prevent this happen? Or to provide an
default icon instead?

In a more general case, how could I prevent
unauthorized error in ZPT?

Rgs
Kent Sin

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