diff --git a/src/calibre/db/cli/cmd_set_metadata.py b/src/calibre/db/cli/cmd_set_metadata.py
index 6e90d2d10b..57ae7232cd 100644
--- a/src/calibre/db/cli/cmd_set_metadata.py
+++ b/src/calibre/db/cli/cmd_set_metadata.py
@@ -137,7 +137,7 @@ def verify_int(x):
             'first argument'
         ))
     if len(args) < 2 and not opts.field:
-        raise SystemExit(_('You must specify either a field or an opf file'))
+        raise SystemExit(_('You must specify either a field or an OPF file'))
     book_id = int(args[0])
 
     if len(args) > 1: