mysqli error

I am getting this error

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1

This is the code I used:

$sql = mysqli_query($con, "SELECT * FROM  ".decrypt($_SESSION["name"], ENCRYPTION_KEY)."_table  ");
while ($row = mysqli_fetch_array($con)) {
   // displays data
}
echo mysqli_error($con);

I don't know what the problem is,but when I do this:echo "SELECT * FROM ".decrypt($_SESSION["name"], ENCRYPTION_KEY)."_table ");

I am getting this:

SELECT * FROM ab_table

The above query is correct,but why does it flag an error?