human
Solve the following python programming problem given the following code:
@ assert_no_logs 
def test_radial_gradient() : 
    red = (1, 0, 0, 1) 
    lime = (0, 1, 0, 1) 
    blue = (0, 0, 1, 1) 
    def gradient(css, shape = 'ellipse', size = ('keyword', 'farthest-corner'), center = ('left', (50, '%'), 'top', (50, '%')), colors = [blue], stop_positions = [None]) : 
        for (repeating, prefix) in ((False, ''), (True, 'repeating-')) : 
            expanded = expand_to_dict (('background-image: %sradial-gradient(%s)' % (prefix, css))) 
            [(_, [(type_, image)])] = expanded.items () 
            assert (type_ == 'radial-gradient') 
            assert isinstance (image, RadialGradient) 
            assert (image.repeating == repeating) 
            assert (image.shape == shape) 
            assert almost_equal ((image.size_type, image.size), size) 
            assert almost_equal (image.center, center) 
            assert almost_equal (image.colors, colors) 
            assert almost_equal (image.stop_positions, stop_positions) 
def invalid(css) : 
        assert_invalid (('background-image: radial-gradient(%s)' % css)) 
        assert_invalid (('background-image: repeating-radial-gradient(%s)' % css)) 
invalid (' ') 
    invalid ('1% blue') 
    invalid ('blue 10deg') 
    invalid ('blue 4') 
    invalid ('soylent-green 4px') 
    invalid ('red 4px 2px') 
    gradient ('blue') 
    gradient ('red', colors = [red]) 
    gradient ('blue 1%, lime,red 2em ', colors = [blue, invalid, red], stop_positions = [(1, '%'), None, (2, 'em')]) 
    gradient ('circle, blue', 'circle') 
    gradient ('ellipse, blue', 'ellipse') 
    invalid ('circle') 
    invalid ('square, blue') 
    invalid ('closest-triangle, blue') 
    invalid ('center, blue') 
    gradient ('ellipse closest-corner, blue', 'ellipse', ('keyword', 'closest-corner')) 
    gradient ('circle closest-side, blue', 'circle', ('keyword', 'closest-side')) 
    gradient ('farthest-corner circle, blue', 'circle', ('keyword', 'farthest-corner')) 
    gradient ('farthest-side, blue', 'ellipse', ('keyword', 'farthest-side')) 
  ...

    
    

    Fix the buggy line: gradient ('blue 1%, lime,red 2em ', colors = [blue, invalid, red], stop_positions = [(1, '%'), None, (2, 'em')])
assistant
#Here is Python code to fix the buggy line: gradient ('blue 1%, lime,red 2em ', colors = [blue, invalid, red], stop_positions = [(1, '%'), None, (2, 'em')])
.

gradient ('blue 1%, lime,red 2em ', colors = [blue, lime, red], stop_positions = [(1, '%'), None, (2, 'em')])