diff --git a/library/pem.c b/library/pem.c
index 969d492..fcfde94 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -343,7 +343,7 @@
     ret = mbedtls_base64_decode( NULL, 0, &len, s1, s2 - s1 );
 
     if( ret == MBEDTLS_ERR_BASE64_INVALID_CHARACTER )
-        return( MBEDTLS_ERR_PEM_INVALID_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PEM_INVALID_DATA, ret ) );
 
     if( ( buf = mbedtls_calloc( 1, len ) ) == NULL )
         return( MBEDTLS_ERR_PEM_ALLOC_FAILED );
@@ -352,7 +352,7 @@
     {
         mbedtls_platform_zeroize( buf, len );
         mbedtls_free( buf );
-        return( MBEDTLS_ERR_PEM_INVALID_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PEM_INVALID_DATA, ret ) );
     }
 
     if( enc != 0 )