diff --git a/tests/test_directions.cpp b/tests/test_directions.cpp
index 5db962c8fafd88b400cb60c894591501e70b0f9e..6d5e4c521abcf6f083400ab61bee404275bce102 100644
--- a/tests/test_directions.cpp
+++ b/tests/test_directions.cpp
@@ -60,7 +60,7 @@ BOOST_AUTO_TEST_CASE(test_radec_to_lmn_single) {
 
   xt::xtensor<double, 2> lmn_actual({2, 3});
   directions.radec2lmn<Directions::SINGLE>(reference, lmn_actual);
-  for (size_t i; i < 2; ++i) {
+  for (size_t i = 0; i < 2; ++i) {
     BOOST_CHECK_CLOSE(lmn_expected(i, 0), lmn_actual(i, 0), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 1), lmn_actual(i, 1), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 2), lmn_actual(i, 2), 1.e-6);
@@ -83,7 +83,7 @@ BOOST_AUTO_TEST_CASE(test_radec_to_lmn_multi) {
 
   xt::xtensor<double, 2> lmn_actual({2, 3});
   directions.radec2lmn<Directions::MULTI>(reference, lmn_actual);
-  for (size_t i; i < 2; ++i) {
+  for (size_t i = 0; i < 2; ++i) {
     BOOST_CHECK_CLOSE(lmn_expected(i, 0), lmn_actual(i, 0), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 1), lmn_actual(i, 1), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 2), lmn_actual(i, 2), 1.e-6);
@@ -107,7 +107,7 @@ BOOST_AUTO_TEST_CASE(test_radec_to_lmn_xsimd) {
   xt::xtensor<double, 2> lmn_actual({2, 3});
   directions.radec2lmn<Directions::computation_strategy::MULTI_SIMD>(
       reference, lmn_actual);
-  for (size_t i; i < 2; ++i) {
+  for (size_t i = 0; i < 2; ++i) {
     BOOST_CHECK_CLOSE(lmn_expected(i, 0), lmn_actual(i, 0), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 1), lmn_actual(i, 1), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 2), lmn_actual(i, 2), 1.e-6);
@@ -134,7 +134,7 @@ BOOST_AUTO_TEST_CASE(test_radec_to_lmn_xsimd_with_remainder) {
   xt::xtensor<double, 2> lmn_actual({3, 3});
   directions.radec2lmn<Directions::computation_strategy::MULTI_SIMD>(
       reference, lmn_actual);
-  for (size_t i; i < 3; ++i) {
+  for (size_t i = 0; i < 3; ++i) {
     BOOST_CHECK_CLOSE(lmn_expected(i, 0), lmn_actual(i, 0), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 1), lmn_actual(i, 1), 1.e-6);
     BOOST_CHECK_CLOSE(lmn_expected(i, 2), lmn_actual(i, 2), 1.e-6);