Sharepoint workflow assignment email doesn't preserve lookups

Question

I am using Sharepoint workflow 2013 and my issue is with assignment email body in the action "Start a task process". Whenever I add lookups using "Add or Change Lookup" button, it is changed to this "{#index of the lookup}", and if i added just one lookup i get the following error.

Using the special characters '[%%]' or '[%xxx%] in any string, or using the special character '{' in a string that also contains a workflow lookup, may corrupt the string and cause an unexpected result when the workflow runs.

I follow these steps:

1. From the drop down on the right of the "start a task process" action, I choose properties.
2. I open "AssignmentEmailBody" string builder. Assignment Email Option
3. I add my lookups to be used the email. LookUp Added
4. I press ok for the string builder window and ok for the properties window.
5. If I opened the properties again I find the lookups replaced. Replaced lookup

I want to preserve the lookups.

Any help would appreciated.