Is NSEC case sensitive while being signed?

[nospam.d.lca at neverbox.com]

I am using dnssec-signzone from BIND 9.5.0b2.  It seems that if I
change the case of the next domain name in the RDATA of NSEC record,
the signature in RRSIG for the NSEC record will change.

Does this mean that next domain name in NSEC is case sensitive, or did
I make some mistake in my experiment?

thank you.
Yue