See your other question for an answer of how to properly get the number of parameters.
The bug you are having could be due to a version of mxnet, there used to be a bug in the print_summary method. Try to upgrade to mxnet 1.3. You can do that by typing directly in your jupyter notebook
!pip uninstall -y mxnet-cu90mkl
!pip install mxnet-cu90mkl
edit: (I confirm your code is running without error in mxnet 1.3.0, though proceed with the adjustements suggested here: Understand number of parameters in mx.viz.print_summary(sym) to get the right number of parameters)