Resharper change my build-in types into .NET type

Hello, everyone.

I'm user resharper 7.x. I encountered an problem, when I move one of my functions to another class, all my build-in types in the destination class are changed to .NET type.
Here's the case:
My original code is:

     private class SourceClass
        {
            public static void Function()
            {
                string s = "";
                string empty = string.Empty;
            }
        }

        public class DestinationClass
        {
            private string str = string.Empty;
            private double num = double.NaN;
        }
After I moved 'Function()' from SourceClass to DestinationClass with resharper's 'Move to Another Type', code changed:

        private class SourceClass
        {
        }

        public class DestinationClass
        {
            private string str = String.Empty;
            private double num = Double.NaN;

            public static void Function()
            {
                string s = "";
                string empty = String.Empty;
            }
        }
All the build-in types are changed to .NET type automatically, which I do not expect.

Does anybody has any clue about it?
How can I configure resharper to prevent it from changing my build-in type?