Missing XML validation issue reported in fortify scan

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C#

Fortify Scan reported Missing XML validation at below line.

XmlReader.Create(memoryStream)

There is no XSD available for input string. I want to validate memoryStream before it is going to XmlReader.Create(memoryStream). Is there any best way to validate memoryStream for XML in below code to satisfy Fortify Scan.

Actual code:

RequestSecurityTokenResponse resp;
            using (MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(tokenstring)))
            using (XmlReader xmlReader = XmlReader.Create(memoryStream))
            {
                WSTrust13ResponseSerializer serializer = new WSTrust13ResponseSerializer();
                WSTrustSerializationContext serializationContext = new WSTrustSerializationContext();
                resp = serializer.ReadXml(xmlReader, serializationContext);
            }

Fortify Says:
Failure to enable validation when parsing XML gives an attacker the opportunity to supply malicious input.

Most successful attacks begin with a violation of the programmer's assumptions. By accepting an XML document without validating it against a DTD or XML schema, the programmer leaves a door open for attackers to provide unexpected, unreasonable, or malicious input. It is not possible for an XML parser to validate all aspects of a document's content; a parser cannot understand the complete semantics of the data. However, a parser can do a complete and thorough job of checking the document's structure and therefore guarantee to the code that processes the document that the content is well-formed.

What I have tried:

Tried to throw error when TryParsToXML(string) is not parsing.