diff --git a/changelog.d/7575.bugfix b/changelog.d/7575.bugfix
new file mode 100644
index 0000000000..0ab5516eb3
--- /dev/null
+++ b/changelog.d/7575.bugfix
@@ -0,0 +1 @@
+Fix str placeholders in an instance of `PrepareDatabaseException`. Introduced in Synapse v1.8.0.
diff --git a/synapse/storage/prepare_database.py b/synapse/storage/prepare_database.py
index 9afc145340..b95434f031 100644
--- a/synapse/storage/prepare_database.py
+++ b/synapse/storage/prepare_database.py
@@ -366,9 +366,8 @@ def _upgrade_existing_database(
         if duplicates:
             # We don't support using the same file name in the same delta version.
             raise PrepareDatabaseException(
-                "Found multiple delta files with the same name in v%d: %s",
-                v,
-                duplicates,
+                "Found multiple delta files with the same name in v%d: %s"
+                % (v, duplicates,)
             )
 
         # We sort to ensure that we apply the delta files in a consistent