Not able to identify nan values using isnan

I am trying to create a mask associated to my motion compensated block. This mask has the value 0 when it finds nan in mcframe and 1 when it does not. Problem is isnan identifies nan values and assigns correct value in mask matrix except from row 90 - 92. My frames are of size 360x640. I have atatched the data for both the matrix mcframe and mask. Below is the code.

x and y are the positions of the block starting from upper left hand corner and going untill the end of rows and columns in both directions

count is variable to keep track of how many 1's I get in my mask.

 mcframe(y, x) = mc_previous_block;
 if isnan(mcframe(y, x))
     mask(y, x) = 0;
 else
     mask(y, x) = 1;
     count = count + 1;
 end