diff --git a/merge_masscode.bas b/merge_masscode.bas
index f4f2101..9c89d0b 100644
--- a/merge_masscode.bas
+++ b/merge_masscode.bas
@@ -4,9 +4,24 @@ $header
 
 Function main(argc as int, argv as pchar ptr)
     dim as JSON_Object Ptr root1, root2
+    dim as string filename1, filename2
+
+    if argc != 3 then
+        fprint stderr, "Usage: ", Appexename$, " <file1.db> <file2.db>"
+        end = 1
+    else
+        filename1 = command$(1)
+        filename2 = command$(2)
+    end if
+
+    checkFileName(filename1)
+    checkFileName(filename2)
+
+    if not exist("output") then mkpath("output")
+
+    root1 = loadJson(filename1)
+    root2 = loadJson(filename2)
 
-    root1 = loadJson("db.json")
-    root2 = loadJson("mac_db.json")
     if root1 and root2 then
         addFolderName(root1)
         addFolderName(root2)
@@ -20,6 +35,13 @@ Function main(argc as int, argv as pchar ptr)
 
 End Function
 
+Sub checkFileName(filename as string)
+    if not exist(filename) then
+        fprint stderr, "ERROR: Could not load ", filename
+        end = 1
+    end if
+End Sub
+
 Function loadJson(filename as string) as JSON_Object Ptr
     dim as JSON_Value Ptr root