diff --git a/gossa.go b/gossa.go
index 2752424..8054290 100644
--- a/gossa.go
+++ b/gossa.go
@@ -232,10 +232,11 @@ func zipRPC(w http.ResponseWriter, r *http.Request) {
 func rpc(w http.ResponseWriter, r *http.Request) {
 	var err error
 	var rpc rpcCall
-	defer exitPath(w, "rpc", rpc)
+	defer exitPath(w, "rpc", &rpc)
 	bodyBytes, err := io.ReadAll(r.Body)
 	check(err)
 	json.Unmarshal(bodyBytes, &rpc)
+	ret := []byte("ok")
 
 	switch rpc.Call {
 	case "mkdirp":
@@ -245,9 +246,8 @@ func rpc(w http.ResponseWriter, r *http.Request) {
 	case "rm":
 		err = os.RemoveAll(enforcePath(rpc.Args[0]))
 	case "sum":
-		var file *os.File
-		enforcedPath := enforcePath(rpc.Args[0])
-		file, err = os.Open(enforcedPath)
+		file, err := os.Open(enforcePath(rpc.Args[0]))
+		check(err)
 		var hash hash.Hash
 		switch rpc.Args[1] {
 		case "md5":
@@ -262,14 +262,12 @@ func rpc(w http.ResponseWriter, r *http.Request) {
 		_, err = io.Copy(hash, file)
 		check(err)
 		checksum := hash.Sum(nil)
-		checksumHex := make([]byte, hex.EncodedLen(len(checksum)))
-		hex.Encode(checksumHex, checksum)
-		w.Write(checksumHex)
-		return
+		ret = make([]byte, hex.EncodedLen(len(checksum)))
+		hex.Encode(ret, checksum)
 	}
 
 	check(err)
-	w.Write([]byte("ok"))
+	w.Write(ret)
 }
 
 func enforcePath(p string) string {
diff --git a/ui/script.js b/ui/script.js
index e419786..41f2e28 100755
--- a/ui/script.js
+++ b/ui/script.js
@@ -153,21 +153,14 @@ function rpc (call, args, cb) {
   xhr.open('POST', location.origin + window.extraPath + '/rpc')
   xhr.setRequestHeader('Content-Type', 'application/json;charset=UTF-8')
   xhr.send(JSON.stringify({ call, args }))
-  // set the callback function (cb) to true to copy the response to the clipboard
-  if (cb === true) {
-    xhr.onload = () => {
-      navigator.clipboard.writeText(xhr.responseText)
-    };
-  } else {
-    xhr.onload = cb
-  }
+  xhr.onload = cb
   xhr.onerror = () => flicker(sadBadge)
 }
 
 const mkdirCall = (path, cb) => rpc('mkdirp', [prependPath(path)], cb)
 const rmCall = (path1, cb) => rpc('rm', [prependPath(path1)], cb)
 const mvCall = (path1, path2, cb) => rpc('mv', [path1, path2], cb)
-const sumCall = (path, type) => {rpc('sum', [prependPath(path), type], true)}
+const sumCall = (path, type, cb) => rpc('sum', [prependPath(path), type], cb)
 
 // File upload
 let totalDone = 0
@@ -669,8 +662,15 @@ function helpOff () {
 
 // checksums
 function getSum (type) {
-  sumCall(getASelected().innerText, type)
+  upBarPc.style.display = 'block'
+  upBarPc.innerText = 'computing checksum...'
+  upBarPc.style.width = '100%'
   sumsOff()
+  sumCall(getASelected().innerText, type, loaded => {
+    navigator.clipboard.writeText(loaded.target.responseText)
+    upBarPc.style.display = 'none'
+    flicker(okBadge)
+  })
 }
 
 const isSumsMode = () => sums.style.display === 'block'
@@ -678,7 +678,7 @@ const isSumsMode = () => sums.style.display === 'block'
 const sumsToggle = () => isSumsMode() ? sumsOff() : sumsOn()
 
 function sumsOn () {
-  if (getASelected().innerText.endsWith('/')) {
+  if (isFolder(getASelected())) {
     alert('cannot checksum a directory')
     return
   }