.SetAttribute omitting part of attribute's prefix string name

I am setting a basic attribute using the XmlElement function SetAttribute(string name, string value).

It appears that my attribute is not fully set. Part of the string name is omitted and I cannot figure it out. I have changed the order of setting the attribute and appending the element but it has no effect. Additionally, in my root, I set an attribute with a similar name style and it works no problem. What am I missing?



C#:

 //"xmlns:xsi" sets correctly
 XmlElement root = docOut.CreateElement("UpdateRequest");
 docOut.AppendChild(root);
 docOut.DocumentElement.SetAttribute("xmlns:xsi", "http://ift.tt/ra1lAU"); 

 //"xsi:type" : 'xsi:' is omitted in output. 
 XmlElement objects = docOut.CreateElement("Objects");
 root.AppendChild(objects);
 objects.SetAttribute("xsi:type", "DataExtensionObject");

Output:

<UpdateRequest xmlns:xsi="http://ift.tt/ra1lAU">
  <Objects type="DataExtensionObject">
  </Objects>
 </UpdateRequest>

Why is this happening?

Thanks.