diff --git a/tests/unittests/tests-libc/tests-libc.c b/tests/unittests/tests-libc/tests-libc.c
index 55835ee1825b6..1d0f27e331cfb 100644
--- a/tests/unittests/tests-libc/tests-libc.c
+++ b/tests/unittests/tests-libc/tests-libc.c
@@ -45,6 +45,7 @@ static void test_libc_swprintf(void)
     TEST_ASSERT_EQUAL_INT(strcmp("Hello World!", string_writer_str(&sw)), 0);
 
     /* check that we can add to the string */
+    /* Writing 10 characters, returns 10 bytes written */
     res = swprintf(&sw, "0123456789");
     TEST_ASSERT_EQUAL_INT(res, 10);
     TEST_ASSERT_EQUAL_INT(strcmp("Hello World!0123456789", string_writer_str(&sw)), 0);