diff --git a/src/EfficientDynamoDb/Internal/Converters/Collections/ArrayDdbConverter.cs b/src/EfficientDynamoDb/Internal/Converters/Collections/ArrayDdbConverter.cs
index 93b119c0..66f8fded 100644
--- a/src/EfficientDynamoDb/Internal/Converters/Collections/ArrayDdbConverter.cs
+++ b/src/EfficientDynamoDb/Internal/Converters/Collections/ArrayDdbConverter.cs
@@ -55,7 +55,7 @@ private AttributeValue WriteInlined(ref T[] value)
             var list = new List<AttributeValue>(value.Length);
 
             for (var i = 0; i < value.Length; i++)
-                list[i] = ElementConverter.Write(ref value[i]);
+                list.Add(ElementConverter.Write(ref value[i]));
 
             return new AttributeValue(new ListAttributeValue(list));
         }
diff --git a/src/EfficientDynamoDb/Internal/Converters/Collections/IReadOnlyCollectionDdbConverter.cs b/src/EfficientDynamoDb/Internal/Converters/Collections/IReadOnlyCollectionDdbConverter.cs
index 1af8950b..31be7c77 100644
--- a/src/EfficientDynamoDb/Internal/Converters/Collections/IReadOnlyCollectionDdbConverter.cs
+++ b/src/EfficientDynamoDb/Internal/Converters/Collections/IReadOnlyCollectionDdbConverter.cs
@@ -54,11 +54,10 @@ private AttributeValue WriteInlined(ref IReadOnlyCollection<T> value)
         {
             var list = new List<AttributeValue>(value.Count);
 
-            var i = 0;
             foreach (var item in value)
             {
                 var itemCopy = item;
-                list[i++] = ElementConverter.Write(ref itemCopy);
+                list.Add(ElementConverter.Write(ref itemCopy));
             }
 
             return new ListAttributeValue(list);