Why are pValue and FStat 'NaN'?

I am new to statistic tool.

1). I use "stepwiselm(ds1,'poly1432')" to perform regression and got some results like:

Removing a:(aw^3), FStat = NaN, pValue = NaN

and Estimate, Se, tStae and pValue in "Estimated Coefficients" are positive numbers which are less than 1.

2) I use "fitlm(ds1,'poly1322')", and got "Estimated Coefficients" like:

                   Estimate     SE    tStat    pValue
                   _________    __    _____    ______

    (Intercept)            0    0      NaN     NaN   
    a                      0    0      NaN     NaN   
    aw                     0    0      NaN     NaN

What does "NaN" mean? who can explain? Thank you very much