diff --git a/gramps/gen/plug/docgen/graphdoc.py b/gramps/gen/plug/docgen/graphdoc.py
index be9fe9f0d..694812168 100644
--- a/gramps/gen/plug/docgen/graphdoc.py
+++ b/gramps/gen/plug/docgen/graphdoc.py
@@ -590,6 +590,7 @@ class GVDocBase(BaseDoc, GVDoc):
 
     def start_subgraph(self, graph_id):
         """ Implement GVDocBase.start_subgraph() """
+        graph_id = graph_id.replace(' ', '_') # for user-defined ID with space
         self.write(
             '  subgraph cluster_%s\n' % graph_id +
             '  {\n' +