PySAL natural breaks from Numpy array

I am attempting to extract natural break values from a raster using Python.

The intended process goes: Arcpy raster object to NumPy Array (using RasterToNumPyArray) to natural break values (using PySAL Natural Breaks function).

import arcpy, pysal
from pysal.esda.mapclassify import Natural_Breaks as nb
# code to create greenIndex arcpy Raster object here
greenArray = arcpy.RasterToNumPyArray(greenIndex)
breaks = nb(greenArray,k=2,initial=20)

This code returns the error, “ValueError: matrix must be 2-dimensional”.

As far as I know, greenArray is a 2-dimensional array. Any hints are greatly appreciated!