NDArray string problem

I try to use
var bb8List = new NDArray(typeof(string) , new Shape(bb8Num));

but it show error like this

Exception has occurred: CLR/System.NotSupportedException
An unhandled exception of type 'System.NotSupportedException' occurred in NumSharp.Core.dll: 'Specified method is not supported.'
at NumSharp.Backends.Unmanaged.ArraySlice.Allocate(Type elementType, Int32 count, Boolean fillDefault)
at NumSharp.Backends.UnmanagedStorage.Allocate(Shape shape, Type dtype, Boolean fillZeros)
at NumSharp.NDArray..ctor(Type dtype, Shape shape)
at TFDemo.Program.Main(String[] args) in /Users/lokinfey/Desktop/File/Proj/AI/tensorflownet/code/TFDemo/Program.cs:line 61