Negative values of the Tsai-Wu failure function?

Hi all,
I am using the following failure function inside my own 2D FE code for analysis of laminated composites:
+S11^2/(Xt*Xc)+S22^2/(Yt*Yc)+S12^2/(XY^2) -S11*S22/sqrt(Xt*Xc*Yt*Yc)
where Xc,Yc>0.
I sometimes get negative F values. Numerically, it seems to be possible to have negative F values, however, since the theory is originated from energy based theories, negative F values do not seem to make any sense.
Any thoughts on this?
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