spec-md edit-server してブラウザ側で編集して保存しようとするとエラーが出て保存されない

[geckotang]

screen-spec-md edit-serverで http://localhost:3001/ が立ち上がったあと、そこの画面で編集を行い、Saveボタンを押したところ、次のエラーが表示されて、保存もされていませんでした。

Error: EISDIR: illegal operation on a directory, open '/Users/geckotang/Work/PROJECT_NAME/markdown'
    at Object.openSync (fs.js:440:3)
    at Object.writeFileSync (fs.js:1265:35)
    at /Users/geckotang/Work/PROJECT_NAME/node_modules/screen-spec-md/server/editable.js:61:10
    at /Users/geckotang/Work/PROJECT_NAME/node_modules/screen-spec-md/server/editable.js:68:7
    at Layer.handle [as handle_request] (/Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/layer.js:95:5)
    at next (/Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/layer.js:95:5)
    at /Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/index.js:281:22
    at Function.process_params (/Users/geckotang/Work/PROJECT_NAME/node_modules/express/lib/router/index.js:335:12)

ディレクトリの状況

drwxr-xr-x geckotang staff   192 B  Thu Nov 28 11:32:23 2019   _spec
drwxr-xr-x geckotang staff    96 B  Thu Nov 28 11:29:39 2019   markdown
drwxr-xr-x geckotang staff  23.9 KB Thu Nov 28 11:23:28 2019   node_modules
.rw-r--r-- geckotang staff 503.6 KB Thu Nov 28 11:23:29 2019   package-lock.json
.rw-r--r-- geckotang staff   706 B  Thu Nov 28 11:29:28 2019   package.json
.rw-r--r-- geckotang staff   957 B  Thu Nov 28 11:17:18 2019   README.md
drwxr-xr-x geckotang staff    96 B  Thu Nov 28 11:28:51 2019   spec

Nodeのバージョン

[geckotang]$ node -v
v12.13.0