Why I can't set value for "SLA Status" this filed?

[Cecilia Chu]

I added a new text field in view screen, named "SLA Status".

It be populated base on "Time to resolution" this field.
I added a post-function in CST this issue type's workflow, When I click resolve transition, It not works, appear error.

Please check the code:

string timeToValue = timeToResolution;
// Get index of last '|'
int index = lastIndexOf(timeToValue, "|");
int length = length(timeToValue);
if(substring(timeToValue, index+1, length) < 0)

{ slaStatus = "Breached"; }

else

{ slaStatus = "Met"; }

I'm looking forward your support!!!