Invalid Length Parameter ....

I have a ##temp table which collects command (varchar(120)) info from sysjobsteps table as follows:
command
------------------------------
BACKUP DATABASE FDMS_11111_2 To Backup_11111_2_4Thu with init, name = 'Backup of Job_11111_2_4Thu'

Now, I need to extract the database name from the command string: It did return the correct database name but ended with error message.

select substring(command, 17, charindex(' To Backup_',Command)-17) from ##tempServerNameDatabaseNameJobHistory

Command
-------
FDMS_11111_2
FDMS_9999_2
(2 row(s) affected)

Server: Msg 536, Level 16, State 3, Line 1 Invalid length parameter passed to the substring function.

-D