maintaining case when pasting into emacs search

[Jai Dayal]

Hi,

When I visit a file, copy something out of it, and then search for this
string in another instance, I do C-s to invoke isearch, and then C-y to
paste what I've copied. However, when I paste it, it comes out in all
lowercase.

1:AAIAAJTJ8o3qZQAAATkCmMTN148=
comes out
1:aaiaajtj.....  etc etc.

Surprisingly, I just pasted it in this email (outside of emacs) and it
pasted with the correct case.

What am I doing wrong?
Jai