Hello, I have the following problem:

Number of obs = 559
F(5, 552) = .
Prob > F = .
R-squared = 0.0860
Root MSE = 5.5101

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Robust
matematicas | Coef. Std. Err. t P>|t| [95% Conf. Interval]
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hbirth | 1.547084 .4992672 3.10 0.002 .5663882 2.52778
wi | 7.405085 1.861222 3.98 0.000 3.74914 11.06103
incentivos | 3.425106 1.395038 2.46 0.014 .6848722 6.165339
sleep | -2.080386 .8880527 -2.34 0.020 -3.824762 -.3360099
eyeglsr3 | 3.967903 .4186677 9.48 0.000 3.145526 4.79028
frqhdr3 | -2.703347 .8265039 -3.27 0.001 -4.326825 -1.079869
_cons | 6.937919 1.650251 4.20 0.000 3.696379 10.17946

As you can see, my F stadistics don´t have a specific value and I don´t know what is the origin of this problem.