indirect function bug found?

Hi,
This is my first post in this forum.

I am using excel 2010 at office an i accidentally notice an "unusual" bug when using indirect function.

You see, i created several sheets named... a1_strip, b1_strip, c1_strip, d1_strip, and so on.

when I use the indiect function:
=indirect("a1_strip" & "!f2")
it works ok... and so when i reference b1_strip, and d1_strip sheets.

but when I use
=indirect("c1_strip" & "!f2")

it yields !REF error.
I tried several sheet name combination... c1_bar, c1_sfs, c1_etc and c1_
it also have same !REF error.

sheet named c1 is ok.

I wonder why... can you explain it?

Thanks.