Legal scholars from Stanford and the University of Chicago urged the Federal Circuit to rule that secret third-party sales don’t qualify as prior art when analyzing patents’ validity, challenging a broader reading of the on-sale bar from a recent district court decision.

The on-sale bar is meant to prevent inventors from extending their monopolies by commercially exploiting their own inventions before filing patent applications, the professors said in an amicus brief filed Tuesday in the US Court of Appeals for the Federal Circuit.

“Because a third-party sale of an invention does not implicate this concern, such sales, on their own, should not bar an inventor from patenting,” wrote Lisa Larrimore Ouellette of Stanford Law School and Jonathan Masur of the University of Chicago Law School.”

Their brief is in support of Cellulose Material Solutions LLC in its appeal of a California federal court’s ruling that invalidated its patent for cellulose-based insulation and packaging materials. The judge treated a confidential sale by another company as prior art in the validity analysis.