Cryptic Compile Error with Generics

I dumbed down my code a lot so that it is easier to see the issue. When I compile, I get the error:

The type 'T' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.IO.UnmanagedMemoryAccessor.Write<T>(long, ref T)'

Code block is below. I suspect it is a simple thing, just because I am unfamiliar with generics. Feedback would be most appreciated.

	class BigArrayGeneric<T>
	{
		private System.IO.MemoryMappedFiles.MemoryMappedFile mmf;
		public BigArrayGeneric()
		{
			mmf = System.IO.MemoryMappedFiles.MemoryMappedFile.CreateNew("MMF_File", 1024);
		}
		public void Initialize(T val)
		{
			Int64 itemsInitialzed = 0;
			using (var accessor = mmf.CreateViewAccessor(itemsInitialzed, 1024))
			{
				accessor.Write(0, true);
				accessor.Write(1, ref val); // Can't make this compile
			}
			return;
		}
	}