nvlink fatal : Could not open input file 'add.o'

Hello,

Sorry for this easy question but I am having a hard time compiling an example in “https://devblogs.nvidia.com/parallelforall/even-easier-introduction-cuda/

when I am running this

$ nvcc add.cu -o add_cuda --keep

error is “nvlink fatal : Could not open input file ‘add.o’”