diff --git a/gobject/gsignal.c b/gobject/gsignal.c
index 77d8f211e..4982d4a21 100644
--- a/gobject/gsignal.c
+++ b/gobject/gsignal.c
@@ -1224,6 +1224,10 @@ g_signal_stop_emission_by_name (gpointer     instance,
  *
  * Also tries the ancestors of the given type.
  *
+ * The type class passed as @itype must already have been instantiated (for
+ * example, using g_type_class_ref()) for this function to work, as signals are
+ * always installed during class initialization.
+ *
  * See g_signal_new() for details on allowed signal names.
  *
  * Returns: the signal's identifying number, or 0 if no signal was found.