Sounds like you may be trying to index into the array using the min or max value instead of the index:
mn = min(data);
minValue = data(mn);
Don't do that. Use both values if you want to get the index. A better way:
[minValue, indexOfMin] = min(data);
[maxValue, indexOfMax] = max(data);
Or you may have used min for the variable
in which case when you came along and tried to do it again, it would have used your min array (a vector is data was a 2-D array) as indexes into your newly created min array. min() is no longer the min function if you replaced it with a variable of the same name. So if the min was now an array of column mins because you stored the mins in an array called min, then doing min(data) will throw that error if any elements of data have negative, zero, or floating points (fractional) values.