diff --git a/library/bignum.c b/library/bignum.c
index 41daf57..c723d56 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1328,7 +1328,8 @@
 }
 
 /*
- * Helper for mbedtls_mpi subtraction
+ * Helper for mbedtls_mpi subtraction.
+ * This function assumes that X->n >= A->n.
  */
 static void mpi_sub_hlp( const mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi *B )
 {