How to resolve "Validation error: The task.json file was not found in contribution task." when validating Azure DevOps extension? - azure-devops

Question

I am attempting to build and validate an Azure DevOps extension that contains a task contribution but it fails to validate and returns "Validation error: The task.json file was not found in contribution task."
This is an example extension I have created that contains two things:
The root vss-extension.json file
A folder called "task" containing the "task.json" file
The command that I am running to validate the extension is as follows. I am running this from the root folder containing the vss-extension.json file.
tfx extension isvalid --publisher my-publisher --extension-id my-id --service-url https://marketplace.visualstudio.com/
vss-extension.json
{
"manifestVersion": 1,
"id": "demo-extension",
"name": "Demo Extension",
"version": "1.0.0",
"publisher": "demo-publisher",
"targets": [
{
"id": "Microsoft.VisualStudio.Services"
}
],
"description": "Demo extension",
"categories": ["Azure Pipelines"],
"files": [
{
"path": "task"
}
],
"contributions": [
{
"id": "build-task",
"type": "ms.vss-distributed-task.task",
"targets": ["ms.vss-distributed-task.tasks"],
"properties": {
"name": "task"
}
}
]
}
task/task.json
{
"$schema": "https://raw.githubusercontent.com/Microsoft/azure-pipelines-task-lib/master/tasks.schema.json",
"id": "a81df1d3-750f-4d60-a8dc-21970f1956e2",
"name": "DemoBuild",
"friendlyName": "Demo task",
"instanceNameFormat": "Demo task",
"description": "Demo task",
"helpMarkDown": "",
"category": "Build",
"author": "Demo Company",
"version": {
"Major": 0,
"Minor": 1,
"Patch": 0
},
"groups": [
{
"name": "someinput",
"displayName": "SomeInput",
"isExpanded": true
}
],
"inputs": [
{
"name": "someinput",
"type": "string",
"label": "Some Input",
"defaultValue": "",
"required": true,
"helpMarkDown": "Input something",
"groupName": "someinput"
}
],
"execution": {
"Node10": {
"target": "index.js"
}
}
}
In addition to the above I have also downloaded and attempted to validate these official Microsoft extensions with the same error message:
https://github.com/microsoft/azure-devops-extension-tasks
https://github.com/microsoft/PR-Metrics
How might I go about fixing this issue please?