What Causes False Positive Results For Morphine While On Hydrocodone?

My drug screen shows a positive for morphine it also shows negative for hydrocodone . I usually take one two times a day hydrocodone 10 acetaminophen and at times codine 300mg acetaminophen 1 or 2 times for chronic pain. My drug screen shows positive for morphine which I do not take and neg for hydrocodone. Can you explain the posotive for morphine which I do not take?