issue.getField() returns null

[wonder wolf]

I'm trying to get a value from a specific field that determines at runtime. I was trying to check for fields by calling issue.getFields(), but it still returns null. The only one kind of method that returns value properly - method with specified in advance field (example:
issue.getSummary();)
Main method

JiraRestClient restClient = getJiraRestClient(); \\custom method for connecting in jira local server
SearchRestClient searchClient = restClient.getSearchClient();
SearchResult searchResult = searchClient.searchJql(jql, -1, 0, fields).claim();
JsonArrayBuilder issueArrayBuilder = Json.createArrayBuilder();
for (Issue issue : searchResult.getIssues()) {
    Issue initIssue = restClient.getIssueClient().getIssue(issue.getKey()).claim();
    JsonObjectBuilder jsonIssueBuilder = Json.createObjectBuilder();
    for (String field : fields) {
        JsonValue jsonValue = getJsonValueForField(initIssue, field);
        jsonIssueBuilder.add(field, jsonValue);
    }
    JsonObject jsonIssue = jsonIssueBuilder.build();
    issueArrayBuilder.add(jsonIssue);
}
return issueArrayBuilder.build();

private static JsonValue getJsonValueForField(Issue issue, String field) {
    IssueField issueField = issue.getField(field);