diff --git a/sqlite_utils/db.py b/sqlite_utils/db.py
index ee401b6..2d76230 100644
--- a/sqlite_utils/db.py
+++ b/sqlite_utils/db.py
@@ -1975,7 +1975,7 @@ class Table(Queryable):
                     {"index_name": index.name},
                 ).fetchall()[0][0]
                 assert index_sql is not None, (
-                    f"Index '{index}' on table '{self.name}' does not have a "
+                    f"Index '{index.name}' on table '{self.name}' does not have a "
                     "CREATE INDEX statement. You must manually drop this index prior to running this "
                     "transformation and manually recreate the new index after running this transformation."
                 )
diff --git a/tests/test_transform.py b/tests/test_transform.py
index 42fc88e..f9859f8 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -633,3 +633,31 @@ def test_transform_with_indexes_errors(fresh_db, transform_params):
         "You must manually drop this index prior to running this transformation"
         in str(excinfo.value)
     )
+
+
+def test_transform_with_unique_constraint_implicit_index(fresh_db):
+    dogs = fresh_db["dogs"]
+    # Create a table with a UNIQUE constraint on 'name', which creates an implicit index
+    fresh_db.execute(
+        """
+        CREATE TABLE dogs (
+            id INTEGER PRIMARY KEY,
+            name TEXT UNIQUE,
+            age INTEGER
+        );
+    """
+    )
+    dogs.insert({"id": 1, "name": "Cleo", "age": 5})
+
+    # Attempt to transform the table without modifying 'name'
+    with pytest.raises(AssertionError) as excinfo:
+        dogs.transform(types={"age": str})
+
+    assert (
+        "Index 'sqlite_autoindex_dogs_1' on table 'dogs' does not have a CREATE INDEX statement."
+        in str(excinfo.value)
+    )
+    assert (
+        "You must manually drop this index prior to running this transformation and manually recreate the new index after running this transformation."
+        in str(excinfo.value)
+    )