Collection Was Modified; Enumeration Operation May Not Execu

Hello everyone,

I am new in VB.net, I have a treeview which first level node is populated with years. I have written some code which should read each node from tree view and add its months from datatabe under the year. I have written two nested For Each loop in vb.net but face the below problem. Please help me to solve the issue.

Error:

Collection Was Modified; Enumeration Operation May Not Execute.

Code:

Dim monthdt As DataTable = getMonthData()

For Each yearNode As TreeNode In TreeView2.Nodes

For Each row As DataRow In monthdt.Rows

Dim month As New TreeNode()

If row("year").ToString() = yearNode.Text Then

month.Text = row("month").ToString()

month.SelectAction = TreeNodeSelectAction.Select

TreeView2.Nodes.Add(month)

End If