diff --git a/tests/unittests/tests-ipv4_addr/tests-ipv4_addr.c b/tests/unittests/tests-ipv4_addr/tests-ipv4_addr.c
index 5bf790dd76ce..1fa153804590 100644
--- a/tests/unittests/tests-ipv4_addr/tests-ipv4_addr.c
+++ b/tests/unittests/tests-ipv4_addr/tests-ipv4_addr.c
@@ -54,7 +54,7 @@ static void test_ipv4_addr_to_str__addr_NULL(void)
 
 static void test_ipv4_addr_to_str__result_NULL(void)
 {
-    ipv4_addr_t a;
+    ipv4_addr_t a = {0};
 
     TEST_ASSERT_NULL(ipv4_addr_to_str(NULL, &a, IPV4_ADDR_MAX_STR_LEN));
 }
diff --git a/tests/unittests/tests-ipv6_addr/tests-ipv6_addr.c b/tests/unittests/tests-ipv6_addr/tests-ipv6_addr.c
index 18b0578207ae..1248f6dfd79b 100644
--- a/tests/unittests/tests-ipv6_addr/tests-ipv6_addr.c
+++ b/tests/unittests/tests-ipv6_addr/tests-ipv6_addr.c
@@ -826,7 +826,7 @@ static void test_ipv6_addr_to_str__addr_NULL(void)
 
 static void test_ipv6_addr_to_str__result_NULL(void)
 {
-    ipv6_addr_t a;
+    ipv6_addr_t a = {0};
 
     TEST_ASSERT_NULL(ipv6_addr_to_str(NULL, &a, IPV6_ADDR_MAX_STR_LEN));
 }