Justin Bieber reportedly claims a foot injury was the cause of his Miami jail wobbling.

The 20-year-old is facing charges of driving under the influence, resisting arrest and driving without a licence stemming from his January arrest in the Florida city.

Jailhouse footage released last month shows Justin urinating in a cup and unsuccessfully attempting to walk in a straight line for sobriety tests.

And according to new case documents obtained by TMZ, the star says his floundering was caused by a hairline fracture in his right foot, an injury sustained in a skating accident that took place three months prior to the January arrest.

The Washington Post reported Justin’s tests showed his blood alcohol content was below the .02 limit for underage drivers, but other analysis proved Xanax and marijuana was present in his system.

And even though Miami-Dade County has a zero-tolerance policy for underage drivers, it appears Justin’s defence is getting stronger by the day.

Initially authorities claimed he reeked of alcohol, which seems unlikely due to the star’s low blood alcohol content. Another officer claimed he caught the star drag racing in public streets, but it appears there isn’t sufficient enough evidence to back that up either.

Justin is also being sued in Miami for an assault case related to his bodyguard. In a deposition earlier this month, he refused to answer questions about his on-again-off-again girlfriend Selena Gomez.

But according to plaintiff Jeffrey Binion’s attorney, Mark DiCowden, it is possible Justin will receive a court subpoena which orders him to answer every question posed to him about Selena.

"I'm looking for Bieber's reappearance in a deposition to be answered truthfully, honestly, and provide his recollections as to incidents that occurred between him and his bodyguards, including the Selena Gomez incident," the lawyer told E! News, noting several motions were filed last week.

The DUI case continues.