Mistakes of law constitute a mistake within the meaning of this article only when it arises from:
1. A misapprehension of the law by all parties, all supposing that they knew and understood it, and all making substantially the same mistake as to the law; or,
2. A misapprehension of the law by one party, of which the others are aware at the time of contracting, but which they do not rectify.
R.L.1910, § 909.