diff --git a/functions-common b/functions-common
index 497bed2..e252139 100644
--- a/functions-common
+++ b/functions-common
@@ -1075,6 +1075,10 @@
     local file_to_parse=""
     local service=""
 
+    if [ $# -ne 1 ]; then
+        die $LINENO "get_packages takes a single, comma-separated argument"
+    fi
+
     if [[ -z "$package_dir" ]]; then
         echo "No package directory supplied"
         return 1