Index: gnome-python-extras-2.19.1-r2.ebuild
===================================================================
--- gnome-python-extras-2.19.1-r2.ebuild	(revision 134)
+++ gnome-python-extras-2.19.1-r2.ebuild	(working copy)
@@ -53,6 +53,10 @@
 		G2CONF="${G2CONF} --with-gtkmozembed=firefox"
 	elif use seamonkey; then
 		G2CONF="${G2CONF} --with-gtkmozembed=seamonkey"
+	else
+		eerror "You must choose gtkmozembed provider."
+		eerror "Try to USE='-firefox xulrunner'"
+		die "one of (xulrunner firefox seamonkey) must be in USE"
 	fi
 }