diff --git a/src/calibre/srv/html_as_json.cpp b/src/calibre/srv/html_as_json.cpp
index d2805b1659..79b504841b 100644
--- a/src/calibre/srv/html_as_json.cpp
+++ b/src/calibre/srv/html_as_json.cpp
@@ -404,7 +404,7 @@ public:
 				} else if (!write_str_literal("}")) return NULL;
 			}
 		}
-		if (!write_str_literal(",\"nsmap\":")) return NULL;
+		if (!write_str_literal(",\"ns_map\":")) return NULL;
 		if (!this->add_nsmap()) return NULL;
 		if (!write_str_literal("}")) return NULL;
 
diff --git a/src/calibre/srv/tests/content.py b/src/calibre/srv/tests/content.py
index 2cc151d77f..b5e3a040d8 100644
--- a/src/calibre/srv/tests/content.py
+++ b/src/calibre/srv/tests/content.py
@@ -248,7 +248,7 @@ class ContentTest(LibraryBaseTest):
             # print(raw.decode('utf-8'))
             data = json.loads(raw)
             self.ae(data['version'], 1)
-            self.ae(tuple(data['nsmap']), nsmap)
+            self.ae(tuple(data['ns_map']), nsmap)
             bc = data['tree']['c'][1]['c']
             self.ae(bc, body_children)