diff --git a/fdroidserver/vmtools.py b/fdroidserver/vmtools.py
index 16f2b605..e489ce4b 100644
--- a/fdroidserver/vmtools.py
+++ b/fdroidserver/vmtools.py
@@ -323,7 +323,7 @@ class FDroidBuildVm:
         boxfile = abspath(boxfile)
         if not isfile(boxfile):
             raise FDroidBuildVmException(
-                'supplied boxfile \'%s\' does not exist', boxfile
+                'supplied boxfile \'%s\' does not exist' % boxfile
             )
         self.vgrnt.box_add(boxname, abspath(boxfile), force=force)