MVVM模式上的TypeInitializeException

System.TypeInitializationException was unhandled
  Message=The type initializer for 'SmartHomeworkOrganizer.ViewModels.MainViewModel' threw an exception.
  Source=SmartHomeworkOrganizer
  TypeName=SmartHomeworkOrganizer.ViewModels.MainViewModel
  StackTrace:
       at SmartHomeworkOrganizer.ViewModels.MainViewModel..ctor()
       at SmartHomeworkOrganizer.App.OnStartup(Object sender, StartupEventArgs e) in C:UsersMohitDocumentsVisual Studio 2010ProjectsSmartHomeworkOrganizerSmartHomeworkOrganizerApp.xaml.cs:line 21
       at System.Windows.Application.OnStartup(StartupEventArgs e)
       at System.Windows.Application.<.ctor>b__0(Object unused)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
       at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.ProcessQueue()
       at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
       at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
       at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.Run()
       at System.Windows.Application.RunDispatcher(Object ignore)
       at System.Windows.Application.RunInternal(Window window)
       at System.Windows.Application.Run(Window window)
       at System.Windows.Application.Run()
       at SmartHomeworkOrganizer.App.Main() in C:UsersMohitDocumentsVisual Studio 2010ProjectsSmartHomeworkOrganizerSmartHomeworkOrganizerobjDebugApp.g.cs:line 0
       at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.ArgumentException
       Message=Default value type does not match type of property 'Score'.
       Source=WindowsBase
       StackTrace:
            at System.Windows.DependencyProperty.ValidateDefaultValueCommon(Object defaultValue, Type propertyType, String propertyName, ValidateValueCallback validateValueCallback, Boolean checkThreadAffinity)
            at System.Windows.DependencyProperty.ValidateMetadataDefaultValue(PropertyMetadata defaultMetadata, Type propertyType, String propertyName, ValidateValueCallback validateValueCallback)
            at System.Windows.DependencyProperty.RegisterCommon(String name, Type propertyType, Type ownerType, PropertyMetadata defaultMetadata, ValidateValueCallback validateValueCallback)
            at System.Windows.DependencyProperty.Register(String name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata, ValidateValueCallback validateValueCallback)
            at System.Windows.DependencyProperty.Register(String name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata)
            at SmartHomeworkOrganizer.ViewModels.MainViewModel..cctor() in C:UsersMohitDocumentsVisual Studio 2010ProjectsSmartHomeworkOrganizerSmartHomeworkOrganizerViewModelsMainViewModel.cs:line 72
       InnerException:

public static readonly DependencyProperty ScoreProperty =
    DependencyProperty.Register("Score", typeof(float), typeof(MainViewModel), new UIPropertyMetadata(0.0));  

public float Score
{
    get { return (float) GetValue(ScoreProperty); }
    set { SetValue(ScoreProperty, value); }
}