Wiremock request body matchingJsonPath throwing exception

Question

I am trying to verify that the POST call made to downstream service has the correct body/value, but getting a null exception.

import static com.github.tomakehurst.wiremock.client.WireMock.equalTo;
import static com.github.tomakehurst.wiremock.client.WireMock.exactly;
import static com.github.tomakehurst.wiremock.client.WireMock.matchingJsonPath;
import static com.github.tomakehurst.wiremock.client.WireMock.postRequestedFor;
import static com.github.tomakehurst.wiremock.client.WireMock.verify;


@Test
void test() throws Exception {
    mockMvc.perform(
            post(...)
            .andExpect(status().isOk());

    await().atMost(Duration.ofSeconds(5)).untilAsserted(() -> verify(exactly(1), postRequestedFor(urlPathMatching("my-url-path"))
            .withRequestBody(matchingJsonPath("$.field1", equalTo("1")))));
}

Fails with this warning and exception

com.github.tomakehurst.wiremock.matching.PathPattern$SubExpressionException: Warning: JSON path expression '$.accountId' failed to match document '' because of error 'json string can not be null or empty'


Caused by: java.lang.IllegalArgumentException: json string can not be null or empty
   at wiremock.com.jayway.jsonpath.internal.Utils.notEmpty(Utils.java:401)
   at wiremock.com.jayway.jsonpath.internal.ParseContextImpl.parse(ParseContextImpl.java:36)
   at wiremock.com.jayway.jsonpath.JsonPath.read(JsonPath.java:550)
   at com.github.tomakehurst.wiremock.matching.MatchesJsonPathPattern.getExpressionResult(MatchesJsonPathPattern.java:122)
   ... 18 more

Instead, if I use expression mapping it works fine.

.withRequestBody(matchingJsonPath("$.[?(@.field1 == 1)]"))));

Whats the correct way to use StringValuePattern?

My json body looks like this:

{
  "field1": "1"
}