An object in OBJ could not be closed or is invalid

If OBJ is an array of interface objects and one of the objects
cannot be disconnected from the instrument, the remaining objects
in the array will be disconnected from the instrument and a warning
will be displayed.

Example:
g = gpib('ni', 0, 2);
fopen(g)
fprintf(g, '*IDN?');
idn = fscanf(g);
fclose(g);