Abaqus/Standard Contact Error: CLEARANCE CANNOT BE NEGATIVE, ZERO OR DEFAULT

Hi,

I am trying to model two 3D beams (modeled by solid elements) in contact using general contact algorithm in abaqus standard. However, I keep getting the following error-

CLEARANCE CANNOT BE NEGATIVE, ZERO OR DEFAULT

I tried the same model with explicit. it works ok. Any idea what I am doing wrong?

Any help will be highly appreciated.

Thanks

Mohammad