diff --git a/datasette/app.py b/datasette/app.py
index 7aa2ac4b..125b4969 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -278,7 +278,9 @@ class Datasette:
         self.crossdb = crossdb
         self.nolock = nolock
         if memory or crossdb or not self.files:
-            self.add_database(Database(self, is_memory=True), name="_memory")
+            self.add_database(
+                Database(self, is_mutable=False, is_memory=True), name="_memory"
+            )
         # memory_name is a random string so that each Datasette instance gets its own
         # unique in-memory named database - otherwise unit tests can fail with weird
         # errors when different instances accidentally share an in-memory database
diff --git a/tests/test_internals_datasette.py b/tests/test_internals_datasette.py
index 1b4732af..a61aac2d 100644
--- a/tests/test_internals_datasette.py
+++ b/tests/test_internals_datasette.py
@@ -59,7 +59,7 @@ async def test_datasette_constructor():
             "route": "_memory",
             "path": None,
             "size": 0,
-            "is_mutable": True,
+            "is_mutable": False,
             "is_memory": True,
             "hash": None,
         }