diff --git a/dev-python/pyblake2/pyblake2-1.0.0.ebuild b/dev-python/pyblake2/pyblake2-1.0.0.ebuild
index 6fa0b27728e13..e755de9de04f8 100644
--- a/dev-python/pyblake2/pyblake2-1.0.0.ebuild
+++ b/dev-python/pyblake2/pyblake2-1.0.0.ebuild
@@ -47,6 +47,11 @@ python_prepare_all() {
 	distutils-r1_python_prepare_all
 }
 
+python_configure_all() {
+	# reported to break stuff, https://bugs.gentoo.org/638428
+	append-flags -fno-tree-vectorize
+}
+
 python_test() {
 	"${EPYTHON}" test/test.py || die "Tests fail with ${EPYTHON}"
 }