diff --git a/tracdap-runtime/python/src/tracdap/rt/_impl/data.py b/tracdap-runtime/python/src/tracdap/rt/_impl/data.py
index ca9bfd7af..5d6b907b0 100644
--- a/tracdap-runtime/python/src/tracdap/rt/_impl/data.py
+++ b/tracdap-runtime/python/src/tracdap/rt/_impl/data.py
@@ -538,7 +538,7 @@ def _coerce_vector(cls, vector: pa.Array, field: pa.Field, pandas_type=None) ->
         if pa.types.is_null(vector.type):
 
             if field.nullable:
-                return pa.array([], type=field.type, size=len(vector))
+                return pa.nulls(size=len(vector), type=field.type)
             else:
                 raise _ex.EDataConformance(f"All null values in non-null field [{field.name}]")