diff --git a/pywps/app/Process.py b/pywps/app/Process.py
index c52c4f260..8eead0a1b 100644
--- a/pywps/app/Process.py
+++ b/pywps/app/Process.py
@@ -457,7 +457,10 @@ def setup_outputs_from_wps_request(self, wps_request):
             for wps_outpt in wps_request.outputs:
 
                 is_reference = wps_request.outputs[wps_outpt].get('asReference', 'false')
-                mimetype = wps_request.outputs[wps_outpt].get('mimetype', None)
+                mimetype = wps_request.outputs[wps_outpt].get('mimetype', '')
+                if not isinstance(mimetype, str):
+                    mimetype = ''
+
                 if is_reference.lower() == 'true':
                     # check if store is supported
                     if self.store_supported == 'false':