diff --git a/pisi/operations.py b/pisi/operations.py
index 62dd19b7..8b0a332f 100644
--- a/pisi/operations.py
+++ b/pisi/operations.py
@@ -275,7 +275,7 @@ def upgrade_base(A = set()):
     if not ctx.get_option('ignore_safety'):
         if ctx.componentdb.has_component('system.base'):
             systembase = set(ctx.componentdb.get_union_comp('system.base').packages)
-            extra_installs = filter(lambda x: not ctx.installdb.is_installed(x), systembase - A)
+            extra_installs = filter(lambda x: not ctx.installdb.is_installed(x), systembase - set(A))
             if extra_installs:
                 ctx.ui.warning(_('Safety switch: Following packages in system.base will be installed: ') +
                                util.strlist(extra_installs))