sql server - Error in the query "Invalid length parameter passed to the LEFT or SUBSTRING function." -

hello in below query facing issues of "invalid length parameter passed left or substring function." can please solve error me can work on query.

select   substring(     a.processinstancedescription,      charindex('http://', a.processinstancedescription ),      charindex('keyinstr', a.processinstancedescription )        - charindex('http://', a.processinstancedescription )),    c.streamname category,   processinstanceappianid jobid,   a.processinstancename,   a.processinstancetargetdate targetdate,   a.processinstancedescription taskdescription,   b.name department,   substring(     processinstancename,      nullif(       patindex('%[0-9][0-9][0-9][0-9][0-9]%',processinstancename),       0),     7) code    internaluseonly..processinstance    join internaluseonly..departments b      on b.keydepartment = a.keydepartmententered      , b.updoperation < 2   join internaluseonly..processstream c      on c.keyprocessstream = a.keyprocessstream      , c.updoperation < 2    processinstancecompleted null   , a.keyprocessstream in (330)    , a.updoperation < 2 

i have attached image above. want fetch urls task description. can correct query or give me exact query perform same

length parameter substring parameter #3. when parameter <0 error occurs. means have problems first occurence of substring, particulary expression:

charindex('keyinstr', a.processinstancedescription )    - charindex('http://', a.processinstancedescription ) 

probably can't find keyinstr in processinstancedescription field , why fails.

how can guard against error depends on want do.

edit

one way deal check if token has been found ('keyinstr'). if not, substitute result desired value, 0, -1, null or else need signal unexpected situation.

case    when charindex('keyinstr', a.processinstancedescription) != 0     charindex('keyinstr', a.processinstancedescription )        - charindex('http://', a.processinstancedescription )   else 0 end 

...or in case better do...

select   case      when charindex('keyinstr', a.processinstancedescription) != 0       substring(         a.processinstancedescription,          charindex('http://', a.processinstancedescription ),          charindex('keyinstr', a.processinstancedescription )            - charindex('http://', a.processinstancedescription ))     else 0 end,    ...