diff --git a/alloc.c b/alloc.c
index fd2e32df8c..12afadfacd 100644
--- a/alloc.c
+++ b/alloc.c
@@ -82,12 +82,17 @@ void *alloc_object_node(void)
 
 static struct alloc_state commit_state;
 
+unsigned int alloc_commit_index(void)
+{
+	static unsigned int count;
+	return count++;
+}
+
 void *alloc_commit_node(void)
 {
-	static int commit_count;
 	struct commit *c = alloc_node(&commit_state, sizeof(struct commit));
 	c->object.type = OBJ_COMMIT;
-	c->index = commit_count++;
+	c->index = alloc_commit_index();
 	return c;
 }
 
diff --git a/cache.h b/cache.h
index cc46be4e0f..c708062df9 100644
--- a/cache.h
+++ b/cache.h
@@ -1354,6 +1354,7 @@ extern void *alloc_commit_node(void);
 extern void *alloc_tag_node(void);
 extern void *alloc_object_node(void);
 extern void alloc_report(void);
+extern unsigned int alloc_commit_index(void);
 
 /* trace.c */
 __attribute__((format (printf, 1, 2)))