TypeError: unsupported operand parent(s) for *: 'Integer Ring' and '

I am using Sage to plot a slope field for x'=-x+80-40*sign(t-6). My coding looks like:

var('t,x')  def sign(t):      if t > 0:          return 1      if t == 0:          return 0      if t < 0:          return -1    plot_slope_field(-x+80-40*sign(t-6), (t, 0, 5), (y,0,5))  

Yet, when I hit evaluate, the error bellow appears

---------------------------------------------------------------------------  TypeError                                 Traceback (most recent call last)  <ipython-input-1-03195771d047> in <module>        8         return -Integer(1)        9   ---> 10 plot_slope_field(-x+Integer(80)-Integer(40)*sign(t-Integer(6)), (t, Integer(0), Integer(5)), (y,Integer(0),Integer(5)))    /home/sc_serv/sage/local/lib/python3.8/site-packages/sage/rings/integer.pyx in sage.rings.integer.Integer.__mul__ (build/cythonized/sage/rings/integer.c:14016)()     1980             return y     1981   -> 1982         return coercion_model.bin_op(left, right, operator.mul)     1983      1984     cpdef _mul_(self, right):    /home/sc_serv/sage/local/lib/python3.8/site-packages/sage/structure/coerce.pyx in sage.structure.coerce.CoercionModel.bin_op (build/cythonized/sage/structure/coerce.c:11304)()     1246         # We should really include the underlying error.     1247         # This causes so much headache.  -> 1248         raise bin_op_exception(op, x, y)     1249      1250     cpdef canonical_coercion(self, x, y):    TypeError: unsupported operand parent(s) for *: 'Integer Ring' and '<class 'NoneType'>'  

How can I fix this to show my graph?

https://stackoverflow.com/questions/66148543/typeerror-unsupported-operand-parents-for-integer-ring-and-class-none February 11, 2021 at 11:55AM