diff --git a/backend/finance/migrations/0034_reportingentry_local_project.py b/backend/finance/migrations/0034_reportingentry_local_project.py
index 52b61f2d..c9c7ccd0 100644
--- a/backend/finance/migrations/0034_reportingentry_local_project.py
+++ b/backend/finance/migrations/0034_reportingentry_local_project.py
@@ -2,6 +2,44 @@ import django.db.models.deletion
 from django.db import migrations, models
 
 
+def rename_external_project_index(apps, schema_editor):
+    """Free the default index name before adding the new local project field.
+
+    PostgreSQL keeps an index name unchanged when its column is renamed.  The
+    old ``project_id`` index would therefore collide with the index Django
+    creates for the new ``project`` foreign key later in this migration.
+    """
+    if schema_editor.connection.vendor != "postgresql":
+        return
+
+    table_name = apps.get_model("finance", "ReportingEntry")._meta.db_table
+    old_name = schema_editor._create_index_name(table_name, ["project_id"], suffix="")
+    new_name = schema_editor._create_index_name(
+        table_name, ["external_project_id"], suffix=""
+    )
+    quoted_old_name = schema_editor.quote_name(old_name)
+    quoted_new_name = schema_editor.quote_name(new_name)
+    schema_editor.execute(
+        f"ALTER INDEX IF EXISTS {quoted_old_name} RENAME TO {quoted_new_name}"
+    )
+
+
+def restore_external_project_index_name(apps, schema_editor):
+    if schema_editor.connection.vendor != "postgresql":
+        return
+
+    table_name = apps.get_model("finance", "ReportingEntry")._meta.db_table
+    old_name = schema_editor._create_index_name(table_name, ["project_id"], suffix="")
+    new_name = schema_editor._create_index_name(
+        table_name, ["external_project_id"], suffix=""
+    )
+    quoted_old_name = schema_editor.quote_name(old_name)
+    quoted_new_name = schema_editor.quote_name(new_name)
+    schema_editor.execute(
+        f"ALTER INDEX IF EXISTS {quoted_new_name} RENAME TO {quoted_old_name}"
+    )
+
+
 def bind_existing_entries(apps, schema_editor):
     ReportingEntry = apps.get_model("finance", "ReportingEntry")
     Binding = apps.get_model("projects", "ProjectExternalBinding")
@@ -31,6 +69,10 @@ class Migration(migrations.Migration):
             old_name="project",
             new_name="external_project",
         ),
+        migrations.RunPython(
+            rename_external_project_index,
+            restore_external_project_index_name,
+        ),
         migrations.AddField(
             model_name="reportingentry",
             name="project",