diff --git a/src/popupwin.c b/src/popupwin.c
index 73b98be..2ebfda2 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -747,4 +747,16 @@
 # endif
     }
 }
+
+    int
+not_in_popup_window()
+{
+    if (bt_popup(curwin->w_buffer))
+    {
+	emsg(_("E994: Not allowed in a popup window"));
+	return TRUE;
+    }
+    return FALSE;
+}
+
 #endif // FEAT_TEXT_PROP