diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 7f7c715..4fe6e48 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -231,6 +231,7 @@
                            mbedtls_mpi_mod_ext_rep ext_rep )
 {
     int ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    int conv_ret = 0;
 
     /* Do our best to check if r and m have been set up */
     if ( r->limbs == 0 || m->limbs == 0 )
@@ -238,12 +239,23 @@
     if ( r->limbs != m->limbs )
         goto cleanup;
 
-    if ( m->int_rep == MBEDTLS_MPI_MOD_REP_MONTGOMERY)
-        ret = mbedtls_mpi_mod_raw_from_mont_rep( r->p, m );
+    if ( m->int_rep == MBEDTLS_MPI_MOD_REP_MONTGOMERY )
+    {
+        conv_ret = mbedtls_mpi_mod_raw_from_mont_rep( r->p, m );
+        if( conv_ret != 0 )
+            goto cleanup;
+    }
 
     ret = mbedtls_mpi_mod_raw_write( r->p, m, buf, buflen, ext_rep );
 
+    if ( m->int_rep == MBEDTLS_MPI_MOD_REP_MONTGOMERY )
+        conv_ret = mbedtls_mpi_mod_raw_to_mont_rep( r->p, m );
+
 cleanup:
+
+    if ( ret == 0 )
+        ret = conv_ret;
+
     return ( ret );
 }
 /* END MERGE SLOT 7 */
diff --git a/tests/suites/test_suite_bignum_mod.function b/tests/suites/test_suite_bignum_mod.function
index 7042ed3..df6bb45 100644
--- a/tests/suites/test_suite_bignum_mod.function
+++ b/tests/suites/test_suite_bignum_mod.function
@@ -187,9 +187,11 @@
 {
     mbedtls_mpi_uint *N = NULL;
     mbedtls_mpi_uint *R = NULL;
+    mbedtls_mpi_uint *R_COPY = NULL;
     unsigned char *r_buff = NULL;
     mbedtls_mpi_mod_modulus m;
     mbedtls_mpi_mod_residue r;
+    mbedtls_mpi_mod_residue r_copy;
     size_t n_limbs, n_bytes, a_bytes;
 
     mbedtls_mpi_mod_modulus_init( &m );
@@ -201,6 +203,7 @@
 
     /* Allocate the memory for intermediate data structures */
     ASSERT_ALLOC( R, n_bytes );
+    ASSERT_ALLOC( R_COPY, n_bytes );
     ASSERT_ALLOC( r_buff, a_bytes );
 
     /* Test that input's size is not greater to modulo's */
@@ -219,11 +222,18 @@
     TEST_EQUAL( 0, mbedtls_mpi_mod_write( &r, &m, r_buff, a_bytes,
                                           endian ) );
 
+    /* Make sure that writing didn't change the value of r */
+    TEST_EQUAL( 0, mbedtls_mpi_mod_residue_setup( &r_copy, &m, R_COPY, n_limbs ) );
+    TEST_EQUAL( 0, mbedtls_mpi_mod_read( &r_copy, &m, input_A->x, input_A->len,
+                                         endian ) );
+    ASSERT_COMPARE( r.p, r.limbs, r_copy.p, r_copy.limbs );
+
     ASSERT_COMPARE( r_buff, a_bytes, input_A->x, a_bytes );
 exit:
     mbedtls_mpi_mod_modulus_free( &m );
     mbedtls_free( N );
     mbedtls_free( R );
+    mbedtls_free( R_COPY );
     mbedtls_free( r_buff );
 }
 /* END_CASE */