diff --git a/hdmf_docutils/doctools/renderrst.py b/hdmf_docutils/doctools/renderrst.py
index b0a683f..309d7a1 100644
--- a/hdmf_docutils/doctools/renderrst.py
+++ b/hdmf_docutils/doctools/renderrst.py
@@ -7,6 +7,7 @@
 from collections import OrderedDict
 from .rst import RSTDocument, RSTTable, RSTSectionLabelHelper
 import os
+import warnings
 
 
 class DataTypeSection(dict):
@@ -328,7 +329,17 @@ def render_namespace(namespace_catalog,
         ns_src_label = "hdmf-type-namespace-src"
         # Create the target doc
         if namespace_name is None:
-            namespace_name = namespace_catalog.default_namespace
+            core = namespace_catalog.core_namespaces
+            if not core:
+                raise ValueError(
+                    "namespace_name not provided and namespace_catalog has no core namespaces"
+                )
+            if len(core) > 1:
+                warnings.warn(
+                    "namespace_name not provided; namespace_catalog has multiple core "
+                    "namespaces %r. Using the first (%r)." % (core, core[0])
+                )
+            namespace_name = core[0]
         curr_namespace = namespace_catalog.get_namespace(namespace_name)
         # Section heading
 
diff --git a/hdmf_docutils/generate_format_docs.py b/hdmf_docutils/generate_format_docs.py
index 5813acb..fb418a7 100644
--- a/hdmf_docutils/generate_format_docs.py
+++ b/hdmf_docutils/generate_format_docs.py
@@ -118,10 +118,10 @@ def load_namespace(namespace_file,
             pass
     # Load the namespace separately if it already exists or we don't have a default type map specified
     if namespace_catalog is None:
-        namespace_catalog = NamespaceCatalog(default_namespace,
-                                             group_spec_cls=group_spec_cls,
+        namespace_catalog = NamespaceCatalog(group_spec_cls=group_spec_cls,
                                              dataset_spec_cls=dataset_spec_cls,
-                                             spec_namespace_cls=spec_namespace_cls)
+                                             spec_namespace_cls=spec_namespace_cls,
+                                             core_namespaces=[default_namespace])
         namespace_catalog.load_namespaces(namespace_file, resolve=resolve)
 
     return namespace_catalog