Helpful post, I found that my error was using else if instead of elif like so:
if [ -z "$VARIABLE1" ]; then
# do stuff
else if [ -z "$VARIABLE2" ]; then
# do other stuff
fi
Fixed it by changing to this:
if [ -z "$VARIABLE1" ]; then
# do stuff
elif [ -z "$VARIABLE2" ]; then
# do other stuff
fi