diff --git a/src/assemble.js b/src/assemble.js
index acf3a04..90f65dd 100644
--- a/src/assemble.js
+++ b/src/assemble.js
@@ -151,9 +151,6 @@ export function assembleNested(subcolumnData, schema, depth = 0) {
       assembleNested(subcolumnData, child, nextDepth)
       const childData = subcolumnData.get(child.path.join('.'))
       if (!childData) throw new Error('parquet struct-like column missing child data')
-      if (child.element.repetition_type === 'OPTIONAL') {
-        flattenAtDepth(childData, depth)
-      }
       struct[child.element.name] = childData
     }
     // remove children
@@ -161,7 +158,9 @@ export function assembleNested(subcolumnData, schema, depth = 0) {
       subcolumnData.delete(child.path.join('.'))
     }
     // invert struct by depth
-    subcolumnData.set(path, invertStruct(struct, depth))
+    const inverted = invertStruct(struct, nextDepth)
+    if (optional) flattenAtDepth(inverted, depth)
+    subcolumnData.set(path, inverted)
     return
   }
   // assert(schema.element.repetition_type !== 'REPEATED')
diff --git a/test/files/nullable.impala.json b/test/files/nullable.impala.json
index 0ff1a46..17afcd1 100644
--- a/test/files/nullable.impala.json
+++ b/test/files/nullable.impala.json
@@ -13,7 +13,7 @@
     [[null, 1, 2, null], [3, null, 4], [], null],
     {"k1": 2, "k2": null},
     [{"k1": 1, "k3": null}, null, {}],
-    {"A":null,"b":[null],"C":{"d":[[{"E":null,"F":null},{"E":10,"F":"aaa"},{"E":null,"F":null},{"E":-10,"F":"bbb"},{"E":null,"F":null}],[{"E":11,"F":"c"},{}],[],null]},"g":{"g1":{"H":{"i":[2.2,null]}},"g2":{"H":{"i":[]}},"g3":{},"g4":{"H":{}},"g5":{"H":{}}}}
+    {"A":null,"b":[null],"C":{"d":[[{"E":null,"F":null},{"E":10,"F":"aaa"},{"E":null,"F":null},{"E":-10,"F":"bbb"},{"E":null,"F":null}],[{"E":11,"F":"c"},null],[],null]},"g":{"g1":{"H":{"i":[2.2,null]}},"g2":{"H":{"i":[]}},"g3":null,"g4":{"H":{}},"g5":{}}}
   ],
   [
     3,
@@ -37,7 +37,7 @@
     null,
     {},
     null,
-    {"A":null,"C":{},"g":{"foo":{"H":{"i":[2.2,3.3]}}}}
+    {"A":null,"g":{"foo":{"H":{"i":[2.2,3.3]}}}}
   ],
   [
     6,
@@ -45,7 +45,7 @@
     null,
     null,
     null,
-    {}
+    null
   ],
   [
     7,
@@ -53,6 +53,6 @@
     [null, [5, 6]],
     {"k1": null, "k3": null},
     null,
-    {"A":7,"b":[2,3,null],"C":{"d":[[],[{}],null]}}
+    {"A":7,"b":[2,3,null],"C":{"d":[[],[null],null]}}
   ]
 ]