diff --git a/docs/cli-reference.rst b/docs/cli-reference.rst
index 2590595..2f4f5a3 100644
--- a/docs/cli-reference.rst
+++ b/docs/cli-reference.rst
@@ -856,6 +856,7 @@ See :ref:`cli_create_table`.
                                 foreign key
       --ignore                  If table already exists, do nothing
       --replace                 If table already exists, replace it
+      --transform               If table already exists, try to transform the schema
       --load-extension TEXT     SQLite extensions to load
       -h, --help                Show this message and exit.
 
diff --git a/sqlite_utils/cli.py b/sqlite_utils/cli.py
index fe57fbe..fe5a402 100644
--- a/sqlite_utils/cli.py
+++ b/sqlite_utils/cli.py
@@ -1445,9 +1445,24 @@ def create_database(path, enable_wal, init_spatialite, load_extension):
     is_flag=True,
     help="If table already exists, replace it",
 )
+@click.option(
+    "--transform",
+    is_flag=True,
+    help="If table already exists, try to transform the schema",
+)
 @load_extension_option
 def create_table(
-    path, table, columns, pk, not_null, default, fk, ignore, replace, load_extension
+    path,
+    table,
+    columns,
+    pk,
+    not_null,
+    default,
+    fk,
+    ignore,
+    replace,
+    transform,
+    load_extension,
 ):
     """
     Add a table with the specified columns. Columns should be specified using
@@ -1482,6 +1497,8 @@ def create_table(
             return
         elif replace:
             db[table].drop()
+        elif transform:
+            pass
         else:
             raise click.ClickException(
                 'Table "{}" already exists. Use --replace to delete and replace it.'.format(
@@ -1489,7 +1506,12 @@ def create_table(
                 )
             )
     db[table].create(
-        coltypes, pk=pk, not_null=not_null, defaults=dict(default), foreign_keys=fk
+        coltypes,
+        pk=pk,
+        not_null=not_null,
+        defaults=dict(default),
+        foreign_keys=fk,
+        transform=transform,
     )
 
 
diff --git a/sqlite_utils/db.py b/sqlite_utils/db.py
index e695304..25fded0 100644
--- a/sqlite_utils/db.py
+++ b/sqlite_utils/db.py
@@ -1689,10 +1689,10 @@ class Table(Queryable):
                     create_table_not_null.add(key)
         elif isinstance(not_null, set):
             create_table_not_null.update((rename.get(k) or k) for k in not_null)
-        elif not_null is None:
+        elif not not_null:
             pass
         else:
-            assert False, "not_null must be a dict or a set or None"
+            assert False, "not_null must be a dict or a set or None, it was {}".format(repr(not_null))
         # defaults=
         create_table_defaults = {
             (rename.get(c.name) or c.name): c.default_value