need to grep a string in a string

Hi I am tryibg to check for presance of a string in a big string. I tried as follows:

IS_VECTOR_TEST_CASE=`grep AIV "$PWD"`
if [ "$IS_VECTOR_TEST_CASES" = "" ]
then
#do something
else
#do something
fi
This always giving me as if [ "$IS_VECTOR_TEST_CASES" = "" ] is true with following error:
grep: /abc/bcd/cde/AIV/def: Invalid argument

Please help me in this issue...