diff --git a/lib/lib-dynamodb/README.md b/lib/lib-dynamodb/README.md
index 9fed58f274b8a..fc88a486c7e1c 100644
--- a/lib/lib-dynamodb/README.md
+++ b/lib/lib-dynamodb/README.md
@@ -105,7 +105,11 @@ export interface marshallOptions {
    */
   convertEmptyValues?: boolean;
   /**
-   * Whether to remove undefined values while marshalling.
+   * Whether to remove undefined values from JS arrays/Sets/objects
+   * when marshalling to DynamoDB lists/sets/maps respectively.
+   *
+   * A DynamoDB item is not itself considered a map. Only
+   * attributes of an item are examined.
    */
   removeUndefinedValues?: boolean;
   /**
diff --git a/packages/util-dynamodb/src/marshall.ts b/packages/util-dynamodb/src/marshall.ts
index 10cc7c4970e21..6e33209051483 100644
--- a/packages/util-dynamodb/src/marshall.ts
+++ b/packages/util-dynamodb/src/marshall.ts
@@ -13,7 +13,11 @@ export interface marshallOptions {
    */
   convertEmptyValues?: boolean;
   /**
-   * Whether to remove undefined values while marshalling.
+   * Whether to remove undefined values from JS arrays/Sets/objects
+   * when marshalling to DynamoDB lists/sets/maps respectively.
+   *
+   * A DynamoDB item is not itself considered a map. Only
+   * attributes of an item are examined.
    */
   removeUndefinedValues?: boolean;
   /**