It is alleged that the banned substance was found in the World No. 15's system after a test at the BMW Open in Munich.
He continued to play and lost to Britain's Andy Murray in the final of the Aegon Championships at Queen's but he has been out of action since pulling out of Wimbledon before his second round match due to a knee injury.
An official from the Croatian Tennis Foundation was quoted as saying that the findings may be a mistake.
And Croatian paper 'Jutarnji List' reported that the 2010 Australian Open semi-finalist produced an adverse finding due to 'incautious use of glucose'.
The story comes just days after Serbian player Viktor Troicki was handed an 18-month ban for failing to provide a blood sample at the Monte Carlo Masters in April.
Troicki is appealing the decision and says he has never taken a prohibited substance.