diff --git a/src/ag-window.c b/src/ag-window.c
index 843f59f..7f1b036 100644
--- a/src/ag-window.c
+++ b/src/ag-window.c
@@ -41,6 +41,8 @@ struct _AgWindowPrivate {
     gchar *uri;
 };
 
+G_DEFINE_QUARK(ag-window-error-quark, ag_window_error);
+
 G_DEFINE_TYPE(AgWindow, ag_window, GTK_TYPE_APPLICATION_WINDOW);
 
 #define GET_PRIVATE(instance) (G_TYPE_INSTANCE_GET_PRIVATE((instance), AG_TYPE_WINDOW, AgWindowPrivate))
diff --git a/src/ag-window.h b/src/ag-window.h
index c0a2d1e..3034bb4 100644
--- a/src/ag-window.h
+++ b/src/ag-window.h
@@ -7,6 +7,11 @@
 
 G_BEGIN_DECLS
 
+typedef enum {
+    AG_WINDOW_ERROR_EMPTY_CHART,
+    AG_WINDOW_ERROR_NO_NAME,
+} AgWindowError;
+
 #define AG_TYPE_WINDOW         (ag_window_get_type())
 #define AG_WINDOW(o)           (G_TYPE_CHECK_INSTANCE_CAST((o), AG_TYPE_WINDOW, AgWindow))
 #define AG_WINDOW_CLASS(k)     (G_TYPE_CHECK_CLASS_CAST((k), AG_TYPE_WINDOW, AgWindowClass))
@@ -35,6 +40,9 @@ void ag_window_update_from_chart(AgWindow *window);
 void ag_window_set_uri(AgWindow *window, const gchar *uri);
 gchar *ag_window_get_uri(AgWindow *window);
 
+#define AG_WINDOW_ERROR (ag_window_error_quark())
+GQuark ag_window_error_quark(void);
+
 G_END_DECLS
 
 #endif /* __AG_WINDOW_H__ */