尝试从Windows certstore读取PrivateKey时发生异常

\'cert.PrivateKey\' threw an exception of type \'System.Security.Cryptography.CryptographicException\'

en System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContainer)
en System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContainer, Int32 dwKeySize, SafeProvHandle& safeProvHandle, SafeKeyHandle& safeKeyHandle)
en System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair()
en System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize)
en System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey()
en ValidKeyDll.ValidKey.getLlaveDeAlmacen(String almacen, Boolean esLlavePrivada) en C:\\Users\\desarrollo\\Documents\\ValidKeyDll\\ValidKeyDll\\ValidKey.cs:línea 58
en ValidKeyDll.ValidKey.firmaCadena(String almacen, String cadenaFirmar) en C:\\Users\\desarrollo\\Documents\\ValidKeyDll\\ValidKeyDll\\ValidKey.cs:línea 117

X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
RSACryptoServiceProvider csp = null;
foreach (X509Certificate2 cert in store.Certificates)
{
   if (cert.Subject.Contains(almacen))
   {
      if (cert.NotAfter.CompareTo(System.DateTime.Now) <= 0)
         throw new CertificadoVencidoException();

      if (isPrivateKey)
         csp = (RSACryptoServiceProvider)cert.PrivateKey;
      else
         csp = (RSACryptoServiceProvider)cert.PublicKey.Key;

      break;
   }
}