Set item Permissions

Hi,

 I am trying to use the set item permissions in order to set the permissions for a specific item in a list. I use the action Set Item Permission.

When I try to give permissions to a specific user I get error occurred. I can successfully break the inheritance or I can successfully inherit from parent but once I put user in the users field I receive the error.

Does anyone had a similar error?

 Thank you in advance,

Ioanna