diff --git a/tests/unittests/tests-core/tests-core-cib.c b/tests/unittests/tests-core/tests-core-cib.c
index 69b1824988eaf..7c5d70ddbfe93 100644
--- a/tests/unittests/tests-core/tests-core-cib.c
+++ b/tests/unittests/tests-core/tests-core-cib.c
@@ -74,46 +74,78 @@ static void test_cib_peek__overflow(void)
 
 static void test_cib_peek_at(void)
 {
+    /* Peeking an empty cib should give an error */
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 0));
 
+    /* Put 1 item in cib and check that we can peek it */
     TEST_ASSERT_EQUAL_INT( 0, cib_put(&cib));
+    TEST_ASSERT_EQUAL_INT( 0, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 0, cib_peek_at(&cib, 0));
+
+    /* Peek past the end should give an error. */
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 1));
 
+    /* Put another item in cib. */
     TEST_ASSERT_EQUAL_INT( 1, cib_put(&cib));
+
+    /* cib should now hold the indices { 0, 1 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 0, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 0, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT( 1, cib_peek_at(&cib, 1));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 2));
 
+    /* Put another item in cib. */
     TEST_ASSERT_EQUAL_INT( 2, cib_put(&cib));
+
+    /* cib should now hold the indices { 0, 1, 2 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 0, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 0, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT( 1, cib_peek_at(&cib, 1));
     TEST_ASSERT_EQUAL_INT( 2, cib_peek_at(&cib, 2));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 3));
 
+    /* Put another item in cib. */
     TEST_ASSERT_EQUAL_INT( 3, cib_put(&cib));
+
+    /* cib should now hold the indices { 0, 1, 2, 3 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 0, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 0, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT( 1, cib_peek_at(&cib, 1));
     TEST_ASSERT_EQUAL_INT( 2, cib_peek_at(&cib, 2));
     TEST_ASSERT_EQUAL_INT( 3, cib_peek_at(&cib, 3));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 4));
 
+    /* Remove an item from cib. */
     TEST_ASSERT_EQUAL_INT( 0, cib_get(&cib));
+
+    /* cib should now hold the indices { 1, 2, 3 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 1, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 1, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT( 2, cib_peek_at(&cib, 1));
     TEST_ASSERT_EQUAL_INT( 3, cib_peek_at(&cib, 2));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 3));
 
+    /* Remove another item from cib. */
     TEST_ASSERT_EQUAL_INT( 1, cib_get(&cib));
+
+    /* cib should now hold the indices { 2, 3 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 2, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 2, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT( 3, cib_peek_at(&cib, 1));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 2));
 
+    /* Remove another item from cib. */
     TEST_ASSERT_EQUAL_INT( 2, cib_get(&cib));
+
+    /* cib should now hold the indices { 3 }. Test this. */
+    TEST_ASSERT_EQUAL_INT( 3, cib_peek(&cib));
     TEST_ASSERT_EQUAL_INT( 3, cib_peek_at(&cib, 0));
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 1));
 
+    /* Remove last item from cib. */
     TEST_ASSERT_EQUAL_INT( 3, cib_get(&cib));
+
+    /* Peeking an empty cib should give an error */
     TEST_ASSERT_EQUAL_INT(-1, cib_peek_at(&cib, 0));
 }