Python Pandas - .isnull() does not work with full dataframe index

Question

I do not understand why i cannot use the method .isnull() for a precise element in my dataframe.

raw_limits['CIRL1_I[X]']

returns

0         1
1        -1
2      0,75
3     -0,75
4       NaN
5       NaN
6    -518,9

Then:

raw_limits['CIRL1_I[X]'].isnull()

returns

0    False
1    False
2    False
3    False
4     True
5     True
6    False

And then finally:

raw_limits['CIRL1_I[X]'][4].isnull()

brings that error

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-88-e89d95cf8aa9> in <module>
      2 measure_points = list(raw_limits.columns)
      3 measure_points.remove(measure_points[0]) # Remove first entry with is "JSN"
----> 4 raw_limits['CIRL1_I[X]'][4].isnull()

AttributeError: 'float' object has no attribute 'isnull'

Can somebody explain me that? Thank you