"The parameter name url is not valid" on SharePoint REST API Call

I'm using the code example 2 found here, attempting to add a file to a list in a SharePoint Online add-in. I'm getting the error "The parameter name url is not valid" in the call to the function:

function addFileToFolder(arrayBuffer) {

        var fileName = $('#itemNumber').val() + "_image.png";

        var fileCollectionEndpoint = serverUrl + "_api/web/Lists/getByTitle('Pictures')/Files/Add(url='" + fileName + "', overwrite=true)"

        console.log(fileCollectionEndpoint);

        return jQuery.ajax({
            url: fileCollectionEndpoint,
            type: "POST",
            data: arrayBuffer,
            processData: false,
            headers: {
                "accept": "application/json;odata=verbose",
                "X-RequestDigest": myFormDigest,
                "content-length": arrayBuffer.byteLength
            }
        });
    }

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fileCollectionEndpoint is:

"https://mySite-4642a56ed1c7b9.sharepoint.com/sites/develop_apps/PunchlistTracking/_api/web/Lists/getByTitle('Pictures')/Files/Add(url='myFile.png', overwrite=true)"

which is a valid API path. I'm not finding anything on this error online. Any ideas?