diff --git a/commit.c b/commit.c
index de16a3c0a2..9a2c7429ec 100644
--- a/commit.c
+++ b/commit.c
@@ -1349,7 +1349,7 @@ void free_commit_extra_headers(struct commit_extra_header *extra)
 	}
 }
 
-int commit_tree(const struct strbuf *msg, unsigned char *tree,
+int commit_tree(const struct strbuf *msg, const unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
 		const char *author, const char *sign_commit)
 {
@@ -1478,7 +1478,7 @@ static const char commit_utf8_warn[] =
 "You may want to amend it after fixing the message, or set the config\n"
 "variable i18n.commitencoding to the encoding your project uses.\n";
 
-int commit_tree_extended(const struct strbuf *msg, unsigned char *tree,
+int commit_tree_extended(const struct strbuf *msg, const unsigned char *tree,
 			 struct commit_list *parents, unsigned char *ret,
 			 const char *author, const char *sign_commit,
 			 struct commit_extra_header *extra)
diff --git a/commit.h b/commit.h
index bd841f4d0c..dd8ae45918 100644
--- a/commit.h
+++ b/commit.h
@@ -231,11 +231,11 @@ struct commit_extra_header {
 extern void append_merge_tag_headers(struct commit_list *parents,
 				     struct commit_extra_header ***tail);
 
-extern int commit_tree(const struct strbuf *msg, unsigned char *tree,
+extern int commit_tree(const struct strbuf *msg, const unsigned char *tree,
 		       struct commit_list *parents, unsigned char *ret,
 		       const char *author, const char *sign_commit);
 
-extern int commit_tree_extended(const struct strbuf *msg, unsigned char *tree,
+extern int commit_tree_extended(const struct strbuf *msg, const unsigned char *tree,
 				struct commit_list *parents, unsigned char *ret,
 				const char *author, const char *sign_commit,
 				struct commit_extra_header *);