XML : XML Signature works with random RSA Key but not with key from certificate

I'm trying to sign an XML Document using the following algorithm in C# :

http://www.w3.org/2001/04/xmldsig-more#rsa-sha256

When I try to sign it with a random RSA Key it works perfectly.

          XmlDocument xmlDoc = new XmlDocument();          xmlDoc.PreserveWhitespace = false;          xmlDoc.Load("hpbtest.xml");          RSA Key = new RSACryptoServiceProvider(2048);          // Create a SignedXml object.          PrefixedSignedXML signedXml = new PrefixedSignedXML(xmlDoc);          // Add the key to the SignedXml document.          signedXml.SigningKey = Key;          signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";          // Create a reference to be signed.          Reference reference = new Reference();          reference.Uri = "#xpointer(//*[@authenticate='true'])";          reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";          // Add an enveloped transformation to the reference.          XmlDsigExcC14NTransform env = new XmlDsigExcC14NTransform();          env.Algorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315";          reference.AddTransform(env);          // Add the reference to the SignedXml object.          signedXml.AddReference(reference);          // Compute the signature.          signedXml.ComputeSignature("ds");          // Get the XML representation of the signature and save          // it to an XmlElement object.          XmlElement xmlDigitalSignature = signedXml.GetXml("ds");          // Append the element to the XML document.          xmlDoc.DocumentElement.AppendChild(xmlDoc.ImportNode(xmlDigitalSignature, true));          xmlDoc.Save("hpbtest.xml");    

But if i want to use an RSA Key from a Certificate :

          XmlDocument xmlDoc = new XmlDocument();          xmlDoc.PreserveWhitespace = false;          xmlDoc.Load("hpbtest.xml");          RSA Key = new GestionCertificat("CN=Bruno").getClePrivee();//Get the private key          // Create a SignedXml object.          PrefixedSignedXML signedXml = new PrefixedSignedXML(xmlDoc);          // Add the key to the SignedXml document.          signedXml.SigningKey = Key;          signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";          // Create a reference to be signed.          Reference reference = new Reference();          reference.Uri = "#xpointer(//*[@authenticate='true'])";          reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";          // Add an enveloped transformation to the reference.          XmlDsigExcC14NTransform env = new XmlDsigExcC14NTransform();          env.Algorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315";          reference.AddTransform(env);          // Add the reference to the SignedXml object.          signedXml.AddReference(reference);          // Compute the signature.          signedXml.ComputeSignature("ds");          // Get the XML representation of the signature and save          // it to an XmlElement object.          XmlElement xmlDigitalSignature = signedXml.GetXml("ds");          // Append the element to the XML document.          xmlDoc.DocumentElement.AppendChild(xmlDoc.ImportNode(xmlDigitalSignature, true));          xmlDoc.Save("hpbtest.xml");    

I get this error :

Invalid specified algorithm

In these two examples, my RSA Keys have the same length (2048) and I have no idea why I get this error.

Thank you !

Thomas