"Request message has unresolved parameters." Calling custom action on Case using web API.

// Hook this function up to the "Email Care Plan" button using the Ribbon Workbench.
SDK.CaseForm.emailCarePlan = function() {
    console.log("mdl: mdl_SDK.CaseForm.js: emailCarePlan().");

    // Refuse to send the email if the daignosis isn't done yet.
    var knowledgeArticle = Xrm.Page.getAttribute("mdl_knowledgearticle").getValue();

    if (knowledgeArticle === undefined || knowledgeArticle == null) {
        alert("This case doesn't yet have a care plan in the portal.");

        return;
    }

    var orgUrl = Xrm.Page.context.getClientUrl();

    var caseId = Xrm.Page.data.entity.getId();
    caseId = caseId.replace("{", "");
    caseId = caseId.replace("}", "");

    // The "mdl_EmailCarePlan" part of this must match the unique name of the custom action and
    // that action must be on cases (incidents).
    var url = orgUrl + "/api/data/v8.1/incidents(" + caseId
            + ")/Microsoft.Dynamics.CRM.mdl_EmailCarePlan";

    console.log("Hitting " + url);

    var data = {
        "dummyParam": "dummyValue" 
    };

    var req = new XMLHttpRequest();

    req.open("POST", url, true);
    req.setRequestHeader("Accept", "application/json");
    req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
    req.setRequestHeader("OData-MaxVersion", "4.0");
    req.setRequestHeader("OData-Version", "4.0");

    req.onreadystatechange = function () {
        if (this.readyState == 4) { // 4: DONE
            req.onreadystatechange = null;

            if (this.status == 200 || this.status == 204) {
                alert("Thank you. The care plan link will be sent shortly.");
            }
            else if (this.status >= 400) {
                var error = JSON.parse(this.response).error;
                alert("Please report this error to support: Email Care Plan button on Case"
                        + " form. " + error.message);
            }
        }
    }

    req.send(window.JSON.stringify(data));
};