diff --git a/python/obitools3/utils.pyx b/python/obitools3/utils.pyx
index 5d5cbdc..acc884f 100644
--- a/python/obitools3/utils.pyx
+++ b/python/obitools3/utils.pyx
@@ -15,7 +15,10 @@ from obitools3.dms.capi.obierrno cimport OBI_LINE_IDX_ERROR, \
                                          OBI_ELT_IDX_ERROR
                                          #obi_errno
 
+#from obitools3.dms.obiseq cimport Nuc_Seq, Nuc_Seq_Stored
 
+
+# TODO RollbackException?
 cdef obi_errno_to_exception(int obi_errno, index_t line_nb=-1, object elt_id=None, str error_message=None) :
     global obi_errno
     if obi_errno > 0 :
@@ -60,6 +63,8 @@ cdef bytes tobytes(object string):
         @return a transcoded string
         @rtype: bytes 
     """
+    if string is None:
+        return None
     if isinstance(string, bytes):
         return string
     return str2bytes(string)
@@ -98,7 +103,7 @@ cdef obitype_t get_obitype_single_value(object value) :
     elif value_type == bool :
         value_obitype = OBI_BOOL        
     elif value_type == str or value_type == bytes :
-        if is_a_DNA_seq(tobytes(value)) :
+        if is_a_DNA_seq(tobytes(value)): #or value_type == Nuc_Seq or value_type == Nuc_Seq_Stored:  # TODO discuss
             value_obitype = OBI_SEQ
         elif len(value) == 1 :
             value_obitype = OBI_CHAR