diff --git a/odb/source-files.c b/odb/source-files.c
index df0ea9ee62..7496e1d9f8 100644
--- a/odb/source-files.c
+++ b/odb/source-files.c
@@ -36,7 +36,7 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	struct odb_source_files *files;
 
 	CALLOC_ARRAY(files, 1);
-	odb_source_init(&files->base, odb, path, local);
+	odb_source_init(&files->base, odb, ODB_SOURCE_FILES, path, local);
 	files->loose = odb_source_loose_new(&files->base);
 	files->packed = packfile_store_new(&files->base);
 
diff --git a/odb/source-files.h b/odb/source-files.h
index 58753d40de..803fa995fb 100644
--- a/odb/source-files.h
+++ b/odb/source-files.h
@@ -30,6 +30,8 @@ void odb_source_files_free(struct odb_source_files *files);
  */
 static inline struct odb_source_files *odb_source_files_downcast(struct odb_source *source)
 {
+	if (source->type != ODB_SOURCE_FILES)
+		BUG("trying to downcast source of type '%d' to files", source->type);
 	return container_of(source, struct odb_source_files, base);
 }
 
diff --git a/odb/source.c b/odb/source.c
index d8b2176a94..c7dcc528f6 100644
--- a/odb/source.c
+++ b/odb/source.c
@@ -13,10 +13,12 @@ struct odb_source *odb_source_new(struct object_database *odb,
 
 void odb_source_init(struct odb_source *source,
 		     struct object_database *odb,
+		     enum odb_source_type type,
 		     const char *path,
 		     bool local)
 {
 	source->odb = odb;
+	source->type = type;
 	source->local = local;
 	source->path = xstrdup(path);
 }
diff --git a/odb/source.h b/odb/source.h
index e6698b73a3..a1f2f8fdb1 100644
--- a/odb/source.h
+++ b/odb/source.h
@@ -1,6 +1,18 @@
 #ifndef ODB_SOURCE_H
 #define ODB_SOURCE_H
 
+enum odb_source_type {
+	/*
+	 * The "unknown" type, which should never be in use. This is type
+	 * mostly exists to catch cases where the type field remains zeroed
+	 * out.
+	 */
+	ODB_SOURCE_UNKNOWN,
+
+	/* The "files" backend that uses loose objects and packfiles. */
+	ODB_SOURCE_FILES,
+};
+
 /*
  * The source is the part of the object database that stores the actual
  * objects. It thus encapsulates the logic to read and write the specific
@@ -19,6 +31,9 @@ struct odb_source {
 	/* Object database that owns this object source. */
 	struct object_database *odb;
 
+	/* The type used by this source. */
+	enum odb_source_type type;
+
 	/*
 	 * Figure out whether this is the local source of the owning
 	 * repository, which would typically be its ".git/objects" directory.
@@ -58,6 +73,7 @@ struct odb_source *odb_source_new(struct object_database *odb,
  */
 void odb_source_init(struct odb_source *source,
 		     struct object_database *odb,
+		     enum odb_source_type type,
 		     const char *path,
 		     bool local);