diff --git a/arrow-avro/src/codec.rs b/arrow-avro/src/codec.rs
index d54c6602dad6..2864677f20bc 100644
--- a/arrow-avro/src/codec.rs
+++ b/arrow-avro/src/codec.rs
@@ -307,18 +307,18 @@ impl AvroDataType {
             }
         }
 
-        // Handle JSON nulls per-spec: allowed only for `null` type or unions with null FIRST
+        // Handle JSON nulls per-spec: allowed only for `null` type or unions with null
         if default_json.is_null() {
             return match self.codec() {
                 Codec::Null => Ok(AvroLiteral::Null),
-                Codec::Union(encodings, _, _) if !encodings.is_empty()
-                    && matches!(encodings[0].codec(), Codec::Null) =>
+                Codec::Union(encodings, _, _)
+                    if encodings.iter().any(|enc| matches!(enc.codec(), Codec::Null)) =>
                     {
                         Ok(AvroLiteral::Null)
                     }
-                _ if self.nullability() == Some(Nullability::NullFirst) => Ok(AvroLiteral::Null),
+                _ if self.nullability().is_some() => Ok(AvroLiteral::Null),
                 _ => Err(ArrowError::SchemaError(
-                    "JSON null default is only valid for `null` type or for a union whose first branch is `null`"
+                    "JSON null default is only valid for `null` type or for a union with a `null` branch"
                         .to_string(),
                 )),
             };
@@ -2946,12 +2946,9 @@ mod tests {
         assert_default_stored(&dt_int_nf, &Value::Null);
         let mut dt_int_ns =
             AvroDataType::new(Codec::Int32, HashMap::new(), Some(Nullability::NullSecond));
-        let err2 = dt_int_ns.parse_and_store_default(&Value::Null).unwrap_err();
-        assert!(
-            err2.to_string()
-                .contains("JSON null default is only valid for `null` type"),
-            "unexpected error: {err2}"
-        );
+        let lit3 = dt_int_ns.parse_and_store_default(&Value::Null).unwrap();
+        assert_eq!(lit3, AvroLiteral::Null);
+        assert_default_stored(&dt_int_ns, &Value::Null);
     }
 
     #[test]
diff --git a/arrow-avro/src/reader/mod.rs b/arrow-avro/src/reader/mod.rs
index aa01f272bfeb..6b2568b42eb6 100644
--- a/arrow-avro/src/reader/mod.rs
+++ b/arrow-avro/src/reader/mod.rs
@@ -2936,6 +2936,7 @@ mod test {
         },"default":{"x":7}}),
                 serde_json::json!({"name":"d_nullable_null","type":["null","int"],"default":null}),
                 serde_json::json!({"name":"d_nullable_value","type":["int","null"],"default":123}),
+                serde_json::json!({"name":"d_nullable_null_second","type":["int","null"],"default":null}),
             ],
         );
         let actual = read_alltypes_with_reader_schema(path, reader_schema);
@@ -2943,7 +2944,7 @@ mod test {
         assert!(num_rows > 0, "skippable_types.avro should contain rows");
         assert_eq!(
             actual.num_columns(),
-            22,
+            23,
             "expected exactly our defaulted fields"
         );
         let mut arrays: Vec<Arc<dyn Array>> = Vec::with_capacity(22);
@@ -3070,6 +3071,10 @@ mod test {
         arrays.push(Arc::new(Int32Array::from_iter_values(std::iter::repeat_n(
             123, num_rows,
         ))));
+        arrays.push(Arc::new(Int32Array::from_iter(std::iter::repeat_n(
+            None::<i32>,
+            num_rows,
+        ))));
         let expected = RecordBatch::try_new(actual.schema(), arrays).unwrap();
         assert_eq!(
             actual, expected,