Tridiagonal Linear Solver

[AMITK]

Hi,
I am using the pddttrf_ function to solve a tridiagonal system of equations. This functions calls functions ppddtrf and ppddtrs in the process.
There is a check in the file "pddttrs.f" which is restricting me to take my choice of number of processors.

IF( LLDB.LT.NB ) THEN
INFO = -( 11*100+6 )
END IF

According to this condition (nprocs%LLDB)==0. Why is this restriction placed as in case of blocked distribution the last nodes will have LLDB < NB.

Thanks
Amit