diff --git a/install.sh b/install.sh
index 97235ea..fc0c3c3 100755
--- a/install.sh
+++ b/install.sh
@@ -1,8 +1,9 @@
 #!/bin/bash -eux
 
-if [[ ! ( ( "$1" == "firefox" && "$#" -eq 1 ) ||
-          ( "$1" == "chrome" && "$#" -eq 2 && ${#2} -eq 32 ) ||
-          ( "$1" == "chromium" && "$#" -eq 2 && ${#2} -eq 32 ) ) ]]; then
+if [[ "$#" -lt 1 || (
+          ! ( ( "$1" == "firefox" && "$#" -eq 1 ) ||
+              ( "$1" == "chrome" && "$#" -eq 2 && ${#2} -eq 32 ) ||
+              ( "$1" == "chromium" && "$#" -eq 2 && ${#2} -eq 32 ) ) ) ]]; then
     echo "Usage: $0 <chrome EXTENSION_ID | chromium EXTENSION_ID | firefox>"
     exit 2
 fi