diff --git a/pyerrors/input/dobs.py b/pyerrors/input/dobs.py
index 498e88d8..92ae3e35 100644
--- a/pyerrors/input/dobs.py
+++ b/pyerrors/input/dobs.py
@@ -652,7 +652,7 @@ def _dobsdict_to_xmlstring_spaces(d, space='  '):
     return o
 
 
-def create_dobs_string(obsl, name, spec='dobs v1.0', origin='', symbol=[], who=None, enstags={}):
+def create_dobs_string(obsl, name, spec='dobs v1.0', origin='', symbol=[], who=None, enstags=None):
     """Generate the string for the export of a list of Obs or structures containing Obs
     to a .xml.gz file according to the Zeuthen dobs format.
 
@@ -678,6 +678,8 @@ def create_dobs_string(obsl, name, spec='dobs v1.0', origin='', symbol=[], who=N
         Provide alternative enstag for ensembles in the form enstags = {ename: enstag}
         Otherwise, the ensemble name is used.
     """
+    if enstags is None:
+        enstags = {}
     od = {}
     r_names = []
     for o in obsl: