Negative Sum of Squares interaction? 2-way ANOVA

Deathfish

Hi, currently I am doing 2-way ANOVA. Strange thing is i checked again and again and I still end up with negative Sum of Squares for interaction.

SSt = 1.3
SSa = 0.303
SSb = 820.295

SSab=-819.458

how is this possible and what does it mean?

(90.4, 90.2) (90.7, 90.6) (90.2, 90.4)
(90.1, 90.3) (90.5, 90.6) (89.9, 90.1)
(90.5, 90.7) (90.8, 90.9) (90.4, 90.1)