diff --git a/src/core/workspace.c b/src/core/workspace.c
index d82158e95..e933995f3 100644
--- a/src/core/workspace.c
+++ b/src/core/workspace.c
@@ -577,6 +577,9 @@ meta_workspace_activate_with_focus (MetaWorkspace *workspace,
   gint num_workspaces, current_space, new_space;
   MetaMotionDirection direction;
 
+  g_return_if_fail (META_IS_WORKSPACE (workspace));
+  g_return_if_fail (meta_workspace_index (workspace) != -1);
+
   meta_verbose ("Activating workspace %d",
                 meta_workspace_index (workspace));