diff --git a/install.sh b/install.sh
index f59d4606..56926d2f 100755
--- a/install.sh
+++ b/install.sh
@@ -35,18 +35,25 @@ function linux_cmake_install {
   exit 1
 }
 
-if [[ $# -gt 1 ]]; then
+function usage {
   echo "Usage: $0 [--clang-completer]"
   exit 0
+}
+
+if [[ $# -gt 1 ]]; then
+  usage
 fi
 
 case "$1" in
   --clang-completer)
     cmake_args='-DUSE_CLANG_COMPLETER=ON'
     ;;
-  *)
+  '')
     cmake_args=''
     ;;
+  *)
+    usage
+    ;;
 esac
 
 if [[ ! -z `which cmake &> /dev/null` ]]; then