Spring Boot: no String-argument constructor/factory method to deserialize from String value

I'm trying to read a response from a rest service with RestTemplate (Spring Boot):

RestTemplate restTemplate = new RestTemplate();
ResponseEntity<RMSendPropertyResponse> response = restTemplate.exchange("https://url", HttpMethod.POST, entity, RMSendPropertyResponse.class);
RMSendPropertyResponse rmResponse = response.getBody();

But when errors array is present on the response:

{
    "message": "Something failed.",
    "success": false,
    "errors": [
        {
            "error_code": "MND_00026",
            "error_value": "",
            "error_description": "field not present"
        },
        {
            "error_code": "VAL_00039",
            "error_value": "0",
            "error_description": "Wrong field"
        }
    ],
    "warnings": null,
    "request_timestamp": "18-07-2017 11:34:46",
    "response_timestamp": "18-07-2017 11:34:46"
}

I always get this error:

2017-07-18 12:29:08.220 WARN 9489 --- [nio-8080-exec-9] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to read HTTP message: org.springframework.http.converter.HttpMessageNotReadableException: Could not read document: Can not construct instance of co.easymatch.portals.rightmove.entities.RMError: no String-argument constructor/factory method to deserialize from String value ('MND_00026') at [Source: java.io.PushbackInputStream@77f5bb5f; line: 1, column: 532] (through reference chain: co.portals.entities.RMSendPropertyResponse["errors"]->java.util.ArrayList[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of co.portals.entities.RMError: no String-argument constructor/factory method to deserialize from String value ('MND_00026') at [Source: java.io.PushbackInputStream@77f5bb5f; line: 1, column: 532] (through reference chain: co.portals.entities.RMSendPropertyResponse["errors"]->java.util.ArrayList[0])

And my classes are...

RMSendPropertyResponse class:

@JsonIgnoreProperties(ignoreUnknown = true)
public class RMSendPropertyResponse extends RMResponse {
    private RMPropertyResponse property;
    private List<RMWarning> warnings;
    private List<RMError> errors;

    public RMSendPropertyResponse() {
    }

    public RMPropertyResponse getProperty() {
        return property;
    }

    public void setProperty(RMPropertyResponse property) {
        this.property = property;
    }

    public List<RMWarning> getWarnings() {
        return warnings;
    }

    public void setWarnings(List<RMWarning> warnings) {
        this.warnings = warnings;
    }

    public List<RMError> getErrors() {
        return errors;
    }

    public void setErrors(List<RMError> errors) {
        this.errors = errors;
    }
}

RMError class:

@JsonIgnoreProperties(ignoreUnknown = true)
public class RMError {
    private String error_code;
    private String error_description;
    private String error_value;

    public RMError() {
    }

    public String getError_code() {
        return error_code;
    }

    public void setError_code(String error_code) {
        this.error_code = error_code;
    }

    public String getError_description() {
        return error_description;
    }

    public void setError_description(String error_description) {
        this.error_description = error_description;
    }

    public String getError_value() {
        return error_value;
    }

    public void setError_value(String error_value) {
        this.error_value = error_value;
    }
}

I can't understand why there isn't a constructor/factory method to deserialize from String value.

Thanks