diff --git a/library/bignum.c b/library/bignum.c
index 231fa66d6..b6503bbff 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -158,9 +158,10 @@ int mbedtls_mpi_shrink( mbedtls_mpi *X, size_t nblimbs )
     if( nblimbs > MBEDTLS_MPI_MAX_LIMBS )
         return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
 
-    /* Actually resize up in this case */
+    /* Actually resize up if there are currently fewer than nblimbs limbs. */
     if( X->n <= nblimbs )
         return( mbedtls_mpi_grow( X, nblimbs ) );
+    /* Now X->n > nblimbs >= 0. */
 
     for( i = X->n - 1; i > 0; i-- )
         if( X->p[i] != 0 )