Symptoms:
The following error message appears when creating a custom column with mathematical function,

"String concatenation (+) requires two strings"

Resolution:
If a User Defined Field is used in the Custom column, then the most likely cause is that the user defined field is 'Text' based. For mathematical calculations the User Defined field should be number based. For example, a timesheet row level User Defined Field is created based on a 'Text' format, users enter a number in it, the function would still fail. You would need to edit the User Defined Field and change the format to Number, However doing so would delete all the historical information stored for that User Defined Field.