diff --git a/macros/sql/union.sql b/macros/sql/union.sql
index ac289e2f..3adf5808 100644
--- a/macros/sql/union.sql
+++ b/macros/sql/union.sql
@@ -100,7 +100,7 @@
             select
 
                 {%- if source_column_name is not none %}
-                cast({{ dbt.string_literal(relation) }} as {{ dbt.type_string() }}) as {{ source_column_name }},
+                cast({{ dbt.string_literal(relation.render()) }} as {{ dbt.type_string() }}) as {{ source_column_name }},
                 {%- endif %}
 
                 {% for col_name in ordered_column_names -%}