diff --git a/target_clickhouse/sinks.py b/target_clickhouse/sinks.py
index f3e30dd..b3f9c3a 100644
--- a/target_clickhouse/sinks.py
+++ b/target_clickhouse/sinks.py
@@ -175,6 +175,8 @@ def _pre_validate_for_string_type(self, record: dict) -> dict:
         for key, value in record.items():
             # Checking if the schema expects a string for this key.
             expected_type = self.schema.get("properties", {}).get(key, {}).get("type")
+            if expected_type is None:
+                continue
             if "string" in expected_type and not isinstance(value, str):
                 # Convert the value to string if it's not already a string.
                 record[key] = (