diff --git a/library/x509_crl.c b/library/x509_crl.c
index edeb39b..ac4fc75 100644
--- a/library/x509_crl.c
+++ b/library/x509_crl.c
@@ -79,7 +79,7 @@
             return( 0 );
         }
 
-        return( MBEDTLS_ERR_X509_INVALID_VERSION + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_VERSION, ret ) );
     }
 
     return( 0 );
@@ -125,7 +125,7 @@
         /* Get enclosing sequence tag */
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         end_ext_data = *p + len;
 
@@ -133,7 +133,7 @@
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &len,
                                           MBEDTLS_ASN1_OID ) ) != 0 )
         {
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
         }
         *p += len;
 
@@ -142,29 +142,29 @@
                                            &is_critical ) ) != 0 &&
             ( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) )
         {
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
         }
 
         /* Data should be octet string type */
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &len,
                 MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         /* Ignore data so far and just check its length */
         *p += len;
         if( *p != end_ext_data )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         /* Abort on (unsupported) critical extensions */
         if( is_critical )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -198,27 +198,27 @@
             ext->p = NULL;
             return( 0 );
         }
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
     }
 
     end = *p + ext->len;
 
     if( end != *p + ext->len )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     while( *p < end )
     {
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         *p += len;
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -364,8 +364,8 @@
     if( len != (size_t) ( end - p ) )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     /*
@@ -377,7 +377,7 @@
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
@@ -421,7 +421,7 @@
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     if( ( ret = mbedtls_x509_get_name( &p, p + len, &crl->issuer ) ) != 0 )
@@ -444,10 +444,10 @@
 
     if( ( ret = mbedtls_x509_get_time( &p, end, &crl->next_update ) ) != 0 )
     {
-        if( ret != ( MBEDTLS_ERR_X509_INVALID_DATE +
-                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) &&
-            ret != ( MBEDTLS_ERR_X509_INVALID_DATE +
-                        MBEDTLS_ERR_ASN1_OUT_OF_DATA ) )
+        if( ret != ( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) ) &&
+            ret != ( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                        MBEDTLS_ERR_ASN1_OUT_OF_DATA ) ) )
         {
             mbedtls_x509_crl_free( crl );
             return( ret );
@@ -486,8 +486,8 @@
     if( p != end )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     end = crl->raw.p + crl->raw.len;
@@ -521,8 +521,8 @@
     if( p != end )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     return( 0 );