diff --git a/dev-libs/jansson/jansson-2.13.1.ebuild b/dev-libs/jansson/jansson-2.13.1.ebuild
index d8f772c1fc156..8b9c7d1b6c5e4 100644
--- a/dev-libs/jansson/jansson-2.13.1.ebuild
+++ b/dev-libs/jansson/jansson-2.13.1.ebuild
@@ -14,6 +14,7 @@ SLOT="0"
 KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~s390 ~sparc ~x86"
 IUSE="doc static-libs"
 
+# dev-libs/jansson-2.13.1[doc]: fails to build with >=sphinx-3.0, #731668
 BDEPEND="doc? ( <dev-python/sphinx-3.0.0 )"
 
 multilib_src_configure() {