diff --git a/datasette/database.py b/datasette/database.py
index ef516e9ed9..f2c980d705 100644
--- a/datasette/database.py
+++ b/datasette/database.py
@@ -169,7 +169,11 @@ async def execute_isolated_fn(self, fn):
                 result = fn(isolated_connection)
             finally:
                 isolated_connection.close()
-                self._all_file_connections.remove(isolated_connection)
+                try:
+                    self._all_file_connections.remove(isolated_connection)
+                except ValueError:
+                    # Was probably a memory connection
+                    pass
             return result
         else:
             # Threaded mode - send to write thread
@@ -230,7 +234,11 @@ def _execute_writes(self):
                         result = e
                     finally:
                         isolated_connection.close()
-                        self._all_file_connections.remove(isolated_connection)
+                        try:
+                            self._all_file_connections.remove(isolated_connection)
+                        except ValueError:
+                            # Was probably a memory connection
+                            pass
                 else:
                     try:
                         result = task.fn(conn)
diff --git a/tests/test_internals_database.py b/tests/test_internals_database.py
index eb842c968e..e05111007a 100644
--- a/tests/test_internals_database.py
+++ b/tests/test_internals_database.py
@@ -1,6 +1,7 @@
 """
 Tests for the datasette.database.Database class
 """
+from datasette.app import Datasette
 from datasette.database import Database, Results, MultipleValues
 from datasette.utils.sqlite import sqlite3
 from datasette.utils import Column
@@ -543,7 +544,12 @@ def inner(conn):
 
 
 @pytest.mark.asyncio
-async def test_execute_isolated(db):
+@pytest.mark.parametrize("disable_threads", (False, True))
+async def test_execute_isolated(db, disable_threads):
+    if disable_threads:
+        ds = Datasette(memory=True, settings={"num_sql_threads": 0})
+        db = ds.add_database(Database(ds, memory_name="test_num_sql_threads_zero"))
+
     # Create temporary table in write
     await db.execute_write(
         "create temporary table created_by_write (id integer primary key)"
@@ -577,6 +583,7 @@ def create_shared_table(conn):
     # ... and a second call to isolated should not see that connection either
     assert not await db.execute_isolated_fn(table_exists_checker("created_by_isolated"))
 
+
 @pytest.mark.asyncio
 async def test_mtime_ns(db):
     assert isinstance(db.mtime_ns, int)