should be it

external/duckdb/test/sql/json/issues/issue19357.test

@@ -0,0 +1,20 @@
+# name: test/sql/json/issues/issue19357.test
+# description: Test issue 19357 - Expected unified vector format of type VARCHAR, but found type INT32
+# group: [issues]
+
+require json
+
+query I
+SELECT TO_JSON({'key_1': 'one'}) AS WITHOUT_KEEP_NULL
+----
+{"key_1":"one"}
+
+query I
+SELECT JSON_OBJECT('key_1', 'one', 'key_2', NULL) AS KEEP_NULL_1
+----
+{"key_1":"one","key_2":null}
+
+statement error
+SELECT JSON_OBJECT('key_1', 'one', NULL, 'two') AS KEEP_NULL_2
+----
+json_object() keys must be VARCHAR