diff --git a/src/shell-window-preview-layout.c b/src/shell-window-preview-layout.c
index 9cf6633b1..8d8596af4 100644
--- a/src/shell-window-preview-layout.c
+++ b/src/shell-window-preview-layout.c
@@ -366,6 +366,9 @@ shell_window_preview_layout_add_window (ShellWindowPreviewLayout *self,
   GHashTableIter iter;
   gpointer value;
 
+  g_return_val_if_fail (SHELL_IS_WINDOW_PREVIEW_LAYOUT (self), NULL);
+  g_return_val_if_fail (META_IS_WINDOW (window), NULL);
+
   priv = shell_window_preview_layout_get_instance_private (self);
 
   g_hash_table_iter_init (&iter, priv->windows);
@@ -425,6 +428,9 @@ shell_window_preview_layout_remove_window (ShellWindowPreviewLayout *self,
   GHashTableIter iter;
   gpointer key, value;
 
+  g_return_if_fail (SHELL_IS_WINDOW_PREVIEW_LAYOUT (self));
+  g_return_if_fail (META_IS_WINDOW (window));
+
   priv = shell_window_preview_layout_get_instance_private (self);
 
   g_hash_table_iter_init (&iter, priv->windows);
@@ -473,6 +479,8 @@ shell_window_preview_layout_get_windows (ShellWindowPreviewLayout *self)
   GHashTableIter iter;
   gpointer value;
 
+  g_return_val_if_fail (SHELL_IS_WINDOW_PREVIEW_LAYOUT (self), NULL);
+
   priv = shell_window_preview_layout_get_instance_private (self);
 
   g_hash_table_iter_init (&iter, priv->windows);