The norm should be set to stop_crit[2:]. The [-3:] is causing the underscore in 'x_L2' to be included and causes an error down the line. https://github.com/yaglm/yaglm/blob/e7e0a7060c5da92df1b49faba9c30a4c5e4dd5f9/yaglm/opt/algo/fista.py#L232