I am working on an automation prototype using Nightwatch js and when I am trying to save screenshot, it is giving below error

Error while running .getScreenshot() protocol action: An unknown
server-side error occurred while processing the command. – An unknown
server-side error occurred while proces...
TEST FAILURE: 1 error during execution; 0 tests failed, 0 passed
(10.032s)
TypeError: The "data" argument must be of type string or an instance
of Buffer, TypedArray, or DataView. Received an instance of Object
at Object.writeFile (fs.js:1487:5)
at FSReqCallback.oncomplete (fs.js:180:23)
FAILED: 1 errors and  1 passed (9.662s)

relevant data for nightwatch.config.js:

 test_settings: {
        "screenshots": {
            "enabled": true, // if you want to keep screenshots
            "path": './screenshots' // save screenshots here
          },

test file data:

.pause(3000).saveScreenshot('./screnshots/test.png');

Nightwatch version: "1.5.1"

please note that due to office policy cannot paste complete testcase data here.