diff --git a/library/constant_time_internal.h b/library/constant_time_internal.h
index 9cc63c2..1e4a3ab 100644
--- a/library/constant_time_internal.h
+++ b/library/constant_time_internal.h
@@ -138,6 +138,7 @@
  * \param B        The right-hand MPI. This must point to an array of limbs
  *                 with the same allocated length as \p A.
  * \param limbs    The number of limbs in \p A and \p B.
+ *                 This must not be 0.
  *
  * \return         The result of the comparison:
  *                 \c 1 if \p A is less than \p B.