Steps to reproduce:
1) Create host with Zabbix agent interface.
2) Create 2 Zabbix Agent items with keys:
vfs.fs.size[/,free]
and
vfs.fs.size[/,total]
3) Create Calculated item.
4) Set parameters for Calculated items like:
Key - calculated_1
Type of information - Numeric (unsigned)
Formula - 100*last(//vfs.fs.size[/,free])/last(//vfs.fs.size[/,total])
5) Turn on server and agent. Wait some time to collect data.
6) Open created Calculated item and try to test item.

Result:
Error message appears.

Expected:
Expected trimmed data (as received and displayed on Latest data page.)
No error message.
All others items type of information can be tested without any errors.