It is possible that the "0" key is not functioning properly. You can test this with the keyboard test:
With the calculator off, press and hold ÷ (divide key) and press ON. A few segments of the display will be lit. Now press each key from the top left moving left to right and then down (NOTE: the ENTER key gets pressed in Both the third and fourth rows.) The display will show a changing pattern of segments as keys are pressed.
If the calculator displays an error OR returns to normal operation/display before the last key is pressed, then there could be a problem with the keyboard.