没有为类型找到合适的构造函数--Jackson ObjectMapper

Question

这是我将地图转换为POJO的代码：

public static <T> T createTObject(Class<T> clazz, Map<String,Object> toConvert){
    final ObjectMapper mapper = new ObjectMapper();
    T obj = null;
    try {
        obj = mapper.convertValue(toConvert, clazz);
    } catch (JsonSyntaxException e) {
        throw new MungoException("Cannot create object because JSON string is malformed");
    } catch(Exception e) {
        throw new MungoException("Some other error occurred when trying to deserialize JSON string. Check if POJO field names match with the Map keys.");
    }
    return obj;     
}

问题是，当我为此方法运行JUnit测试时，它会抛出此错误：

java.lang.IllegalArgumentException: No suitable constructor found for type [simple type, class com.mungoae.MungoTest$Greeting]: can not instantiate from JSON object (need to add/enable type information?)
 at [Source: N/A; line: -1, column: -1]

任何人都可以建议我修复此错误吗？

更新

这是抛出此错误的测试：

@Test
public void testMapDBObjectToPOJO(){
    Map<String,Object> map = new HashMap<String,Object>();
    map.put("greeting", "Hello world!");
    map.put("_id", "51cc93ad23187afa8e0a4433");
    Greeting greeting = Mapper.createTObject(Greeting.class, map);
    assertNotNull(greeting);
    assertEquals("Hello world!", greeting.getGreeting());
}