diff --git a/midx.c b/midx.c
index 24e1e72175..6c01f0fa52 100644
--- a/midx.c
+++ b/midx.c
@@ -24,7 +24,7 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
-const unsigned char *get_midx_checksum(struct multi_pack_index *m)
+const unsigned char *get_midx_checksum(const struct multi_pack_index *m)
 {
 	return m->data + m->data_len - m->source->odb->repo->hash_algo->rawsz;
 }
diff --git a/midx.h b/midx.h
index 6e54d73503..7c7e0b5912 100644
--- a/midx.h
+++ b/midx.h
@@ -85,7 +85,7 @@ struct multi_pack_index {
 #define MIDX_EXT_BITMAP "bitmap"
 #define MIDX_EXT_MIDX "midx"
 
-const unsigned char *get_midx_checksum(struct multi_pack_index *m);
+const unsigned char *get_midx_checksum(const struct multi_pack_index *m);
 void get_midx_filename(struct odb_source *source, struct strbuf *out);
 void get_midx_filename_ext(struct odb_source *source, struct strbuf *out,
 			   const unsigned char *hash, const char *ext);