diff --git a/src/test/java/org/cqfn/astranaut/core/base/HoleTest.java b/src/test/java/org/cqfn/astranaut/core/base/HoleTest.java
index 81fcdfd..867dff0 100644
--- a/src/test/java/org/cqfn/astranaut/core/base/HoleTest.java
+++ b/src/test/java/org/cqfn/astranaut/core/base/HoleTest.java
@@ -47,11 +47,19 @@ void testBaseInterface() {
             hole.getProperties().getOrDefault("color", "")
         );
         Assertions.assertEquals(1, hole.getNumber());
+        Assertions.assertEquals("IntegerLiteral<#1>", hole.toString());
         final Type type = hole.getType();
         final String typename = "IntegerLiteral";
         Assertions.assertEquals(typename, type.getName());
         Assertions.assertEquals(typename, type.getHierarchy().get(0));
         Assertions.assertFalse(type.getProperties().containsKey("abracadabra"));
         Assertions.assertEquals(typename, node.getTypeName());
+        boolean oops = false;
+        try {
+            type.createBuilder();
+        } catch (final UnsupportedOperationException ignored) {
+            oops = true;
+        }
+        Assertions.assertTrue(oops);
     }
 }
diff --git a/src/test/java/org/cqfn/astranaut/core/base/MutableNodeTest.java b/src/test/java/org/cqfn/astranaut/core/base/MutableNodeTest.java
index 4c25201..4e6747f 100644
--- a/src/test/java/org/cqfn/astranaut/core/base/MutableNodeTest.java
+++ b/src/test/java/org/cqfn/astranaut/core/base/MutableNodeTest.java
@@ -32,9 +32,6 @@
  * @since 1.0
  */
 class MutableNodeTest {
-    /**
-     * Testing the transformation from 'typical' node to mutable.
-     */
     @Test
     void testBaseInterface() {
         final Node left = LittleTrees.createVariable("x");
@@ -59,4 +56,17 @@ void testBaseInterface() {
         Assertions.assertFalse(result instanceof MutableNode);
         Assertions.assertTrue(mutable.deepCompare(result));
     }
+
+    @Test
+    void testBadTransformation() {
+        final Node left = LittleTrees.createIntegerLiteral(2);
+        final Node right = LittleTrees.createIntegerLiteral(3);
+        final Node original = LittleTrees.createAddition(left, right);
+        final MutableNode mutable = new MutableNode(original);
+        Assertions.assertTrue(
+            mutable.replaceChild(left, LittleTrees.createReturnStatement(null))
+        );
+        final Node result = mutable.rebuild();
+        Assertions.assertSame(DummyNode.INSTANCE, result);
+    }
 }