diff --git a/target_elasticsearch/sinks.py b/target_elasticsearch/sinks.py
index 1536482..4e35a9c 100644
--- a/target_elasticsearch/sinks.py
+++ b/target_elasticsearch/sinks.py
@@ -286,11 +286,17 @@ def create_indices(self, indices: Set[str]) -> None:
                         # Some properties may be problematic - it's the case for fields called "properties",
                         # which usually cause more problems than it solves -> disable dynamic mapping, and
                         # store the field without indexing the contents
+                        # Same for customFields which (eg. used for Ashby), which may be arbitrary
                         "properties": {
                             "properties": {
                                 "type": "object",
                                 "dynamic": "false",
                                 "enabled": False
+                            },
+                            "customFields": {
+                                "type": "object",
+                                "dynamic": "false",
+                                "enabled": False
                             }
                         }
                     }