why it is showing " TypeError: Invalid shape (20,) for image data "

    import numpy as np
    import matplotlib.pyplot as plt  
    m1=np.random.randint(0,20,4*5)
    m1.reshape(4,5)
    plt.imshow(m1)

**when I have executed above code in python 3 on jupyter notebook, I got Type error anyone please answer to this error in simplest way ** like TypeError Traceback (most recent call last) in ----> 1 plt.imshow(m1)

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\pyplot.py in imshow(X, cmap, norm, aspect, interpolation, alpha, vmin, vmax, origin, extent, shape, filternorm, filterrad, imlim, resample, url, data, **kwargs)
   2649         filternorm=filternorm, filterrad=filterrad, imlim=imlim,
   2650         resample=resample, url=url, **({"data": data} if data is not
-> 2651         None else {}), **kwargs)
   2652     sci(__ret)
   2653     return __ret

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\__init__.py in inner(ax, data, *args, **kwargs)
   1563     def inner(ax, *args, data=None, **kwargs):
   1564         if data is None:
-> 1565             return func(ax, *map(sanitize_sequence, args), **kwargs)
   1566 
   1567         bound = new_sig.bind(ax, *args, **kwargs)

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\cbook\deprecation.py in wrapper(*args, **kwargs)
    356                 f"%(removal)s.  If any parameter follows {name!r}, they "
    357                 f"should be pass as keyword, not positionally.")
--> 358         return func(*args, **kwargs)
    359 
    360     return wrapper

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\cbook\deprecation.py in wrapper(*args, **kwargs)
    356                 f"%(removal)s.  If any parameter follows {name!r}, they "
    357                 f"should be pass as keyword, not positionally.")
--> 358         return func(*args, **kwargs)
    359 
    360     return wrapper

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\axes\_axes.py in imshow(self, X, cmap, norm, aspect, interpolation, alpha, vmin, vmax, origin, extent, shape, filternorm, filterrad, imlim, resample, url, **kwargs)
   5613                               resample=resample, **kwargs)
   5614 
-> 5615         im.set_data(X)
   5616         im.set_alpha(alpha)
   5617         if im.get_clip_path() is None:

c:\users\jaiprakash\appdata\local\programs\python\python37-32\lib\site-packages\matplotlib\image.py in set_data(self, A)
    697                 or self._A.ndim == 3 and self._A.shape[-1] in [3, 4]):
    698             raise TypeError("Invalid shape {} for image data"
--> 699                             .format(self._A.shape))
    700 
    701         if self._A.ndim == 3:

TypeError: Invalid shape (20,) for image data