Unable to cast object of type 'Server.Screen' to type 'Server.Form1'

Unable to cast object of type 'Server.Screen' to type 'Server.Form1'

does any one know why i'm getting this error : Unable to cast object of
type 'Server.Screen' to type 'Server.Form1'.
Here is where i'm getting the error :
foreach (Form1 F in Application.OpenForms)
{
F.AddRow(Datas[1], Datas[2] + "/" + Datas[3], Datas[4], Datas[5],
Datas[6], Datas[7], Datas[8], Datas[9], Datas[10], Datas[11] +" Mb",
Datas[12]);
F.AddBot(1);
}
Screen is a window form, but the strange thing is that i get this error
rarely, so i don't know why :S