diff --git a/ui_tree.go b/ui_tree.go
index 28c5adc..fe7439d 100644
--- a/ui_tree.go
+++ b/ui_tree.go
@@ -89,15 +89,12 @@ func (b *Barnard) TreeItemBuild(item uiterm.TreeItem) []uiterm.TreeItem {
 
 	var treeItem TreeItem
 	if ti, ok := item.(TreeItem); !ok {
-		root := b.Client.Channels[0]
+		var root *gumble.Channel
+		b.Client.Do(func() { root = b.Client.Channels[0] })
 		if root == nil {
 			return nil
 		}
-		return []uiterm.TreeItem{
-			TreeItem{
-				Channel: root,
-			},
-		}
+		return []uiterm.TreeItem{TreeItem{Channel: root}}
 	} else {
 		treeItem = ti
 	}
@@ -108,11 +105,17 @@ func (b *Barnard) TreeItemBuild(item uiterm.TreeItem) []uiterm.TreeItem {
 
 	users := []uiterm.TreeItem{}
 	ul := []*gumble.User{}
-	for _, user := range treeItem.Channel.Users {
-		ul = append(ul, user)
-		var u = ul[len(ul)-1]
-		_ = u
-	}
+	cl := []*gumble.Channel{}
+	// TCP handlers mutate both maps; snapshot them while Client.Do holds its
+	// read lock, then sort/render outside the protocol critical section.
+	b.Client.Do(func() {
+		for _, user := range treeItem.Channel.Users {
+			ul = append(ul, user)
+		}
+		for _, subchannel := range treeItem.Channel.Children {
+			cl = append(cl, subchannel)
+		}
+	})
 	sort.Slice(ul, func(i, j int) bool {
 		return ul[i].Name < ul[j].Name
 	})
@@ -123,10 +126,6 @@ func (b *Barnard) TreeItemBuild(item uiterm.TreeItem) []uiterm.TreeItem {
 	}
 
 	channels := []uiterm.TreeItem{}
-	cl := []*gumble.Channel{}
-	for _, subchannel := range treeItem.Channel.Children {
-		cl = append(cl, subchannel)
-	}
 	sort.Slice(cl, func(i, j int) bool {
 		return cl[i].Name < cl[j].Name
 	})
diff --git a/ui_tree_test.go b/ui_tree_test.go
new file mode 100644
index 0000000..69db060
--- /dev/null
+++ b/ui_tree_test.go
@@ -0,0 +1,22 @@
+package main
+
+import (
+	"testing"
+
+	"git.stormux.org/storm/barnard/gumble/gumble"
+)
+
+// Regression: rebuilding the channel tree ranged protocol-owned maps without
+// Client.Do while TCP handlers could add or remove users/channels.
+func TestTreeItemBuildReadsMapsUnderClientSnapshot(t *testing.T) {
+	root := &gumble.Channel{ID: 0, Users: gumble.Users{}, Children: gumble.Channels{}}
+	user := &gumble.User{Session: 1, Name: "user"}
+	child := &gumble.Channel{ID: 2, Name: "child", Users: gumble.Users{}, Children: gumble.Channels{}}
+	root.Users[user.Session] = user
+	root.Children[child.ID] = child
+	b := &Barnard{Client: &gumble.Client{Channels: gumble.Channels{0: root}}, MutedChannels: map[uint32]bool{}}
+	items := b.TreeItemBuild(TreeItem{Channel: root})
+	if len(items) != 2 {
+		t.Fatalf("got %d items", len(items))
+	}
+}