diff --git a/reana_server/rest/workflows.py b/reana_server/rest/workflows.py
index 231118cc..f228ee22 100644
--- a/reana_server/rest/workflows.py
+++ b/reana_server/rest/workflows.py
@@ -765,7 +765,8 @@ def get_workflow_specification(workflow_id_or_name, user):  # noqa
             jsonify(
                 {
                     "specification": workflow.reana_specification,
-                    "parameters": workflow.input_parameters,
+                    # `input_parameters` can be null, if so return an empty dict
+                    "parameters": workflow.input_parameters or {},
                 }
             ),
             200,