interior-point algorithm error - symmetric indefinite factorization

Sometimes when I run the interior-point algorithm with fmincon I get an error occurring in

"backsolveSys (line 18)" in "solveAugSystem (line 23)" in "computeProjResidual (line 150)" in "projConjGrad (line 38)" in "tangentialStep (line 36)" in "computeTrialStep (line 104)" in "barrier (line 348)."

The error message is in Japanese (since i am in Tokyo), but it translates into English as "when doing symmetric indefinite factorization an error occurred." Does anyone know what this means, what could be causing it, and what I can do about it? Thanks.