diff --git a/include/mcl/ec.hpp b/include/mcl/ec.hpp
index 7ae672a..d8a0fc1 100644
--- a/include/mcl/ec.hpp
+++ b/include/mcl/ec.hpp
@@ -427,6 +427,7 @@ void addJacobi(E& R, const E& P, const E& Q)
 /*
 	accept P == Q
 	https://github.com/apache/incubator-milagro-crypto-c/blob/fa0a45a3/src/ecp.c.in#L767-L976
+	(x, y, z) is zero <=> x = 0, y = 1, z = 0
 */
 template<class E>
 void addCTProj(E& R, const E& P, const E& Q)
diff --git a/test/ec_test.cpp b/test/ec_test.cpp
index 1aa241f..f544714 100644
--- a/test/ec_test.cpp
+++ b/test/ec_test.cpp
@@ -499,20 +499,19 @@ struct Test {
 		Fp y(para.gy);
 		Ec P(x, y), Q, R, Zero;
 		Zero.clear();
+		Zero.y = 1;
 		mcl::ec::addCTProj(Q, P, P);
 		Ec::add(R, P, P);
 		CYBOZU_TEST_EQUAL(Q, R);
 		mcl::ec::addCTProj(Q, Q, P);
 		Ec::add(R, R, P);
 		CYBOZU_TEST_EQUAL(Q, R);
-/*
 		mcl::ec::addCTProj(Q, Q, Zero);
 		Ec::add(R, R, Zero);
 		CYBOZU_TEST_EQUAL(Q, R);
 		mcl::ec::addCTProj(Q, Zero, Q);
 		Ec::add(R, Zero, R);
 		CYBOZU_TEST_EQUAL(Q, R);
-*/
 		mcl::ec::addCTProj(Q, Zero, Zero);
 		Ec::add(R, Zero, Zero);
 		CYBOZU_TEST_EQUAL(Q, R);