diff --git a/changelog.d/6449.bugfix b/changelog.d/6449.bugfix
new file mode 100644
index 000000000000..002f33c4507d
--- /dev/null
+++ b/changelog.d/6449.bugfix
@@ -0,0 +1 @@
+Fix error when using synapse_port_db on a vanilla synapse db.
diff --git a/scripts/synapse_port_db b/scripts/synapse_port_db
index 0d3321682c6e..f24b8ffe6741 100755
--- a/scripts/synapse_port_db
+++ b/scripts/synapse_port_db
@@ -782,7 +782,10 @@ class Porter(object):
     def _setup_state_group_id_seq(self):
         def r(txn):
             txn.execute("SELECT MAX(id) FROM state_groups")
-            next_id = txn.fetchone()[0] + 1
+            curr_id = txn.fetchone()[0]
+            if not curr_id:
+                return
+            next_id = curr_id + 1
             txn.execute("ALTER SEQUENCE state_group_id_seq RESTART WITH %s", (next_id,))
 
         return self.postgres_store.runInteraction("setup_state_group_id_seq", r)