diff --git a/backend/address2UPRN/main.py b/backend/address2UPRN/main.py
index 02eb27dc..136f4344 100644
--- a/backend/address2UPRN/main.py
+++ b/backend/address2UPRN/main.py
@@ -510,6 +510,17 @@ def handler(event, context, local=False):
             # Create results DataFrame
             result_df = pd.DataFrame(results_data)
 
+            # The UPRN is integer-valued, but the no-match rows append None, so the
+            # mixed column lands as float64 and would serialise as "100020933699.0".
+            # Coerce to a nullable integer so it round-trips as "100020933699"
+            # (empty when missing) — the form the finaliser and the combined-results
+            # UI expect. `to_numeric(errors="coerce")` also folds the
+            # "invalid postcode" sentinel + blanks to NA (read back as missing).
+            if "address2uprn_uprn" in result_df.columns:
+                result_df["address2uprn_uprn"] = pd.to_numeric(
+                    result_df["address2uprn_uprn"], errors="coerce"
+                ).astype("Int64")
+
             # Save results to S3
             try:
                 save_results_to_s3(result_df, str(task_id), str(subtask_id))