Template Error list index out of range

[whitespirit]

Hello, since one of the last upgrades of calibre it gives me this error (Template Error list index out of range) when calculating this custom column. It is strange that the value in the column is calculated right aftrer a time, but using this value in a combined field yields this error. I didn't change anything so I checked an older version again and in 5.16 it still worked errorfree with the same library and formula.
value for custiom colum :
program: format_number(divide(booksize(), 1048000), '{0:0.1f}')
I use the older version as a tempory work around to create an actualized catalog.
Maybe this can be solved again in a future update.
thank you