issue with grep command

Thu Sep 3 14:51:20 EDT 2009

in tcsh terminal i have the following line of code
cat fileName.txt | grep "a"
which extracts all the instances of "a" from the file in the
terminal..
however when i am trying to create a txt while, i am unable to do so
for some reason..this is the syntaxt i am using --
cat fileName.txt | frep "a" > a.txt
file then appears in the folder, however, it can no be opened because
"the file is not in the right format"..
does anybody know how to correct this problem?
thanks so much