I couple of suggestions how to trouble shoot this issue: 1) Break the call "x = kp_data[0:93, 955].ngims.he_density" into multiple calls in order to determine which part of the call might be failing. For example: temp = kp[0:93,955] print, temp[0:5] ;confirm this is working temp_gims = temp.ngims print, tem_gims[0:5] x = tem_gims.he_density print, x[0:5] 2) My guess is that the variable "x" must be defined with something other than NaNs in another part of the code. I would recommend placing a break point in the code and then stepping through it line by line while monitoring this variable to determine where it is being defined.