diff --git a/decorate.c b/decorate.c
index d9668d2ef..82d9e221e 100644
--- a/decorate.c
+++ b/decorate.c
@@ -6,13 +6,13 @@
 #include "object.h"
 #include "decorate.h"
 
-static unsigned int hash_obj(struct object *obj, unsigned int n)
+static unsigned int hash_obj(const struct object *obj, unsigned int n)
 {
 	unsigned int hash = *(unsigned int *)obj->sha1;
 	return hash % n;
 }
 
-static void *insert_decoration(struct decoration *n, struct object *base, void *decoration)
+static void *insert_decoration(struct decoration *n, const struct object *base, void *decoration)
 {
 	int size = n->size;
 	struct object_decoration *hash = n->hash;
@@ -44,7 +44,7 @@ static void grow_decoration(struct decoration *n)
 	n->nr = 0;
 
 	for (i = 0; i < old_size; i++) {
-		struct object *base = old_hash[i].base;
+		const struct object *base = old_hash[i].base;
 		void *decoration = old_hash[i].decoration;
 
 		if (!base)
@@ -55,7 +55,8 @@ static void grow_decoration(struct decoration *n)
 }
 
 /* Add a decoration pointer, return any old one */
-void *add_decoration(struct decoration *n, struct object *obj, void *decoration)
+void *add_decoration(struct decoration *n, const struct object *obj,
+		void *decoration)
 {
 	int nr = n->nr + 1;
 
@@ -65,7 +66,7 @@ void *add_decoration(struct decoration *n, struct object *obj, void *decoration)
 }
 
 /* Lookup a decoration pointer */
-void *lookup_decoration(struct decoration *n, struct object *obj)
+void *lookup_decoration(struct decoration *n, const struct object *obj)
 {
 	int j;
 
diff --git a/decorate.h b/decorate.h
index 1fa4ad9be..e7328044f 100644
--- a/decorate.h
+++ b/decorate.h
@@ -2,7 +2,7 @@
 #define DECORATE_H
 
 struct object_decoration {
-	struct object *base;
+	const struct object *base;
 	void *decoration;
 };
 
@@ -12,7 +12,7 @@ struct decoration {
 	struct object_decoration *hash;
 };
 
-extern void *add_decoration(struct decoration *n, struct object *obj, void *decoration);
-extern void *lookup_decoration(struct decoration *n, struct object *obj);
+extern void *add_decoration(struct decoration *n, const struct object *obj, void *decoration);
+extern void *lookup_decoration(struct decoration *n, const struct object *obj);
 
 #endif