diff --git a/library/bignum.c b/library/bignum.c
index ba03988..a68957a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -968,17 +968,15 @@
     carry = mbedtls_mpi_core_sub( X->p, A->p, B->p, n );
     if( carry != 0 )
     {
-        /* Propagate the carry to the first nonzero limb of X. */
-        for( ; n < X->n && X->p[n] == 0; n++ )
-            --X->p[n];
-        /* If we ran out of space for the carry, it means that the result
-         * is negative. */
-        if( n == X->n )
+        /* Propagate the carry through the rest of X. */
+        carry = mbedtls_mpi_core_sub_int( X->p + n, X->p + n, carry, X->n - n );
+
+        /* If we have further carry/borrow, the result is negative. */
+        if( carry != 0 )
         {
             ret = MBEDTLS_ERR_MPI_NEGATIVE_VALUE;
             goto cleanup;
         }
-        --X->p[n];
     }
 
     /* X should always be positive as a result of unsigned subtractions. */