why my code does not throw exception as expected?

@RunWith(RobolectricTestRunner.class)
public class ContractExampleTest {
    @Test
    public void testPreconditions() {
        assertThrows(PreconditionError.class, () -> {
            // Given...
            Dictionary<Integer> dict = new Dictionary<>(1);

            dict.put("foo", 1);

            // When...
            dict.put("bar", 2);

            // Then... PreconditionError, not enough capacity!
        });
    }

    @Invariant({
            "capacity() >= count()",
            "count() >= 0"
    })
    static
    class Dictionary<TValue> {
        private int _capacity;
        private Map<String, TValue> _contents;

        public int capacity() {
            return _capacity;
        }

        public int count() {
            return _contents.size();
        }

        @Requires("capacity > 0")
        @Ensures({
                "capacity() > 0",
                "count() == 0"
        })
        public Dictionary(int capacity) {
            this._capacity = capacity;
            this._contents = new HashMap<>(capacity);
        }

        @Requires({
                "key != null",
                "key.length() > 0",
                "count() < capacity()"
        })
        @Ensures({
                "get(key) == value",
                "count() == old(count()) + 1"
        })
        public void put(String key, TValue value) {
            _contents.put(key, value);
        }
    }
}

Hi, in testPreconditions() I inserts twice, so the "capacity() >= count()" in preConditionTest should be violated Because capacity() returns 1, count() returns 2.

But when I executed the test, I got "expected com.google.java.contract.PreconditionError to be thrown, but nothing was thrown."

I have no idea why nothing was thrown. Error message is "expected com.google.java.contract.PreconditionError to be thrown, but nothing is thrown"

Do anyone have some ideas?

Thank you!