Visual Basic Basit Bir Sorun

Dim Oran As New Random
        Dim Yuzdelık As Integer = Oran.Next(70, 100)
        If Yuzdelık >= 90 Then
            MsgBox("1" + Yuzdelık + "a")
        ElseIf Yuzdelık < 90 Then
            MsgBox("2" + Yuzdelık + "b")
        End If

Message="a" dizesinden 'Double' türüne yapılan dönüştürme geçerli değil.

System.InvalidCastException was unhandled
  HResult=-2147467262
  Message="a" dizesinden 'Double' türüne yapılan dönüştürme geçerli değil.
  Source=Microsoft.VisualBasic
  StackTrace:
       konum: Microsoft.VisualBasic.CompilerServices.Conversions.ToDouble(String Value, NumberFormatInfo NumberFormat)
       konum: Microsoft.VisualBasic.CompilerServices.Conversions.ToDouble(String Value)
       konum: WindowsApplication1.Form1.Button1_Click(Object sender, EventArgs e) C:\Users\Sarısoy\Desktop\Basic Keylogger\WindowsApplication1\Form1.vb içinde: satır 7
       konum: System.Windows.Forms.Control.OnClick(EventArgs e)
       konum: System.Windows.Forms.Button.OnClick(EventArgs e)
       konum: System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       konum: System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       konum: System.Windows.Forms.Control.WndProc(Message& m)
       konum: System.Windows.Forms.ButtonBase.WndProc(Message& m)
       konum: System.Windows.Forms.Button.WndProc(Message& m)
       konum: System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       konum: System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       konum: System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       konum: System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       konum: System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       konum: System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       konum: System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       konum: System.Windows.Forms.Application.Run(ApplicationContext context)
       konum: Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
       konum: Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
       konum: Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
       konum: WindowsApplication1.My.MyApplication.Main(String[] Args) 17d14f5c-a337-4978-8281-53493378c1071.vb içinde: satır 81
       konum: System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       konum: System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       konum: Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       konum: System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       konum: System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       konum: System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       konum: System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       konum: System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.FormatException
       HResult=-2146233033
       Message=Giriş dizesi doğru biçimde değildi.
       Source=Microsoft.VisualBasic
       StackTrace:
            konum: Microsoft.VisualBasic.CompilerServices.Conversions.ParseDouble(String Value, NumberFormatInfo NumberFormat)
            konum: Microsoft.VisualBasic.CompilerServices.Conversions.ToDouble(String Value, NumberFormatInfo NumberFormat)
       InnerException: