handling blanks - problem

Hello,

I am trying to assign number of image depending on the value of the nps score. If version is unstable, no nps value is calculated and grey image (num 1) should be assigned. However, DAX calculates blank as infinity so i am having weird (1+4) outcome for the image num.

Color20 = SUMX( VALUES('Circle tbl'[Circle]) , 
SWITCH( 'Circle tbl'[Circle], 
"green", IF( [nps st/nonst] > 20 , 4, 0 ),
"red", IF( [nps st/nonst] <= 10 , 2, 0 ),
"orange", IF( AND( [nps st/nonst] > 10, [nps st/nonst] <= 20 ), 3, 0),
"grey", IF( [nps st/nonst] = "--" , 1 , 0 ),
0
))