Why "The property '__deferred' does not exist on type 'SP.SecurableObject'" message appears when trying to update a sharepoint list item?

Question

I know that updating an item in sharepoint list by using REST API should be very easy, but for some reason, I have not been able to do it. I keep getting "400 Bad Request: The property __deferred does not exist on type SP.SecurableObject error, and I do not know, nor understand, why is that happening. I'm sure this has to be something easy to fix, but I'm a complete newbie in javascript (and in general to everything related to web developing). I have tried some approaches that I find throughout the web, but I was not able to solve this issue.

This is the code that I'm using:

function UpdateItem(ListName, ItemID, ItemData) {

    return $.ajax({
        method: "POST",
        data: JSON.stringify(ItemData),
        headers: {
            "Accept": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val(),
            "X-HTTP-Method": "MERGE",
            "If-Match": ItemData.__metadata.etag,
            "content-type": "application/json;odata=verbose",
        },
        url: ItemData.__metadata.uri,
        success: function () {
            console.log("Item Updated");
        },
        error: function (msgError) {
            console.log("Error");
        }
    });
}

And I'm making use of that function this way:

GetItem(ListName, ItemId).done(function(data){
    var oldItem = data.d;
    oldItem.Title = "Modified Title";
    UpdateItem(ListName, ItemId, oldItem).done(function (data) {
        console.log("Item Updated", data);
    });
});

I'm able to perform an update using jsom, but I want to do it by using REST API.

Could you guys please help me to solve (or/and understand why?) this?

Thaks in advance.