Lookup field onchange event not firing in Dynamics CRM 2013

Hi,

I have lookup field and combobox (optionset) on my form. Lookup field is cleared, if i choose one of values in combobox (I have handler on onchange event). If I put in the lookup field same value as before, the onchange event is not fired. If I put other value, everything is OK.

Lookup is cleared with this code:

Xrm.Page.getAttribute("lookupId").setValue(null);

Any ideas, why onchange event is not fired, when I put same value in the lookup field after deleting it?

Thanks.