truncate only is not a valid option when shrinking a log. If you don't specify a size, it will shrink as far as possible, probably requiring you to grow it again.
Don't run that command.
If you choose to shrink, shrink to a sensible size which is sufficient for your day to day operation.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability