This question comes up first in the Google search for this type error, but does not have a general answer about the cause of the error. The poster’s unique problem was the use of an inappropriate object type as the main argument for plt.imshow(). A more general answer is that plt.imshow() wants an array of floats and if you don’t specify a float, numpy, pandas, or whatever else, might infer a different data type somewhere along the line. You can avoid this by specifying a float for the dtype argument is the constructor of the object.
See the Numpy documentation here.
See the Pandas documentation here