Magnet field: exception from G4magInt_Driver stepsize underflow


I would like to understand what the origin of this exception is:

-------- WWWW ------- G4Exception-START -------- WWWW -------

*** G4Exception : GeomField1001
issued by : G4MagInt_Driver::OneGoodStep()
Stepsize underflow in Stepper !

  • Step’s start x=27365.9 and end x= 27365.9 are equal !!
    Due to step-size= 1.13848e-12. Note that input step was 0.013671
    *** This is just a warning message. ***
    -------- WWWW -------- G4Exception-END --------- WWWW -------

At this x there is no detector and there should not be any field as it is out of the world volume, which is 6m. A user defined magnetic field is used. The field is applied via

G4Mag_SpinEqRhs *equation = new G4Mag_SpinEqRhs(MagneticField::GetInstance());
G4MagIntegratorStepper stepper = new G4ClassicalRK4(equation, 12); // 12: mag field
G4MagInt_Driver driver = new G4MagInt_Driver
mm, stepper, stepper->GetNumberOfVariables());
finder = new G4ChordFinder(driver);
fFieldManager = new G4FieldManager(MagneticField::GetInstance());
fFieldManager->SetDeltaOneStep(1e-10); // improved tracking precision

Do you have any idea how to debug this?

Thank you very much in advance,