diff --git a/moto/dynamodb2/models/__init__.py b/moto/dynamodb2/models/__init__.py
index 7218fe0c9..bcc70ab01 100644
--- a/moto/dynamodb2/models/__init__.py
+++ b/moto/dynamodb2/models/__init__.py
@@ -128,10 +128,10 @@ class Item(BaseModel):
             new_value = list(update_action["Value"].values())[0]
             if action == "PUT":
                 # TODO deal with other types
-                if isinstance(new_value, list):
-                    self.attrs[attribute_name] = DynamoType({"L": new_value})
-                elif isinstance(new_value, set):
+                if set(update_action["Value"].keys()) == set(["SS"]):
                     self.attrs[attribute_name] = DynamoType({"SS": new_value})
+                elif isinstance(new_value, list):
+                    self.attrs[attribute_name] = DynamoType({"L": new_value})
                 elif isinstance(new_value, dict):
                     self.attrs[attribute_name] = DynamoType({"M": new_value})
                 elif set(update_action["Value"].keys()) == set(["N"]):
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index 0e0fcb082..cfba7b15b 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -5683,3 +5683,24 @@ def test_transact_get_items_should_return_empty_map_for_non_existent_item():
     items.should.have.length_of(2)
     items[0].should.equal({"Item": item})
     items[1].should.equal({})
+
+
+@mock_dynamodb2
+def test_dynamodb_update_item_fails_on_string_sets():
+    dynamodb = boto3.resource("dynamodb", region_name="eu-west-1")
+    client = boto3.client("dynamodb", region_name="eu-west-1")
+
+    table = dynamodb.create_table(
+        TableName="test",
+        KeySchema=[{"AttributeName": "record_id", "KeyType": "HASH"},],
+        AttributeDefinitions=[{"AttributeName": "record_id", "AttributeType": "S"},],
+        BillingMode="PAY_PER_REQUEST",
+    )
+    table.meta.client.get_waiter("table_exists").wait(TableName="test")
+    attribute = {"test_field": {"Value": {"SS": ["test1", "test2"],}, "Action": "PUT"}}
+
+    client.update_item(
+        TableName="test",
+        Key={"record_id": {"S": "testrecord"}},
+        AttributeUpdates=attribute,
+    )