diff --git a/include/git2/sys/merge.h b/include/git2/sys/merge.h
index 031941042..eed106c07 100644
--- a/include/git2/sys/merge.h
+++ b/include/git2/sys/merge.h
@@ -36,23 +36,23 @@ GIT_EXTERN(git_merge_driver *) git_merge_driver_lookup(const char *name);
 typedef struct git_merge_driver_source git_merge_driver_source;
 
 /** Get the repository that the source data is coming from. */
-GIT_EXTERN(git_repository *) git_merge_driver_source_repo(
+GIT_EXTERN(const git_repository *) git_merge_driver_source_repo(
 	const git_merge_driver_source *src);
 
 /** Gets the ancestor of the file to merge. */
-GIT_EXTERN(git_index_entry *) git_merge_driver_source_ancestor(
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_ancestor(
 	const git_merge_driver_source *src);
 
 /** Gets the ours side of the file to merge. */
-GIT_EXTERN(git_index_entry *) git_merge_driver_source_ours(
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_ours(
 	const git_merge_driver_source *src);
 
 /** Gets the theirs side of the file to merge. */
-GIT_EXTERN(git_index_entry *) git_merge_driver_source_theirs(
+GIT_EXTERN(const git_index_entry *) git_merge_driver_source_theirs(
 	const git_merge_driver_source *src);
 
 /** Gets the merge file options that the merge was invoked with */
-GIT_EXTERN(git_merge_file_options *) git_merge_driver_source_file_options(
+GIT_EXTERN(const git_merge_file_options *) git_merge_driver_source_file_options(
 	const git_merge_driver_source *src);
 
 
diff --git a/src/merge_driver.c b/src/merge_driver.c
index 5ee0b38b5..0f35d23c2 100644
--- a/src/merge_driver.c
+++ b/src/merge_driver.c
@@ -32,31 +32,31 @@ static struct merge_driver_registry merge_driver_registry;
 
 static void git_merge_driver_global_shutdown(void);
 
-git_repository* git_merge_driver_source_repo(const git_merge_driver_source *src)
+const git_repository* git_merge_driver_source_repo(const git_merge_driver_source *src)
 {
 	assert(src);
 	return src->repo;
 }
 
-git_index_entry* git_merge_driver_source_ancestor(const git_merge_driver_source *src)
+const git_index_entry* git_merge_driver_source_ancestor(const git_merge_driver_source *src)
 {
 	assert(src);
 	return src->ancestor;
 }
 
-git_index_entry* git_merge_driver_source_ours(const git_merge_driver_source *src)
+const git_index_entry* git_merge_driver_source_ours(const git_merge_driver_source *src)
 {
 	assert(src);
 	return src->ours;
 }
 
-git_index_entry* git_merge_driver_source_theirs(const git_merge_driver_source *src)
+const git_index_entry* git_merge_driver_source_theirs(const git_merge_driver_source *src)
 {
 	assert(src);
 	return src->theirs;
 }
 
-git_merge_file_options* git_merge_driver_source_file_options(const git_merge_driver_source *src)
+const git_merge_file_options* git_merge_driver_source_file_options(const git_merge_driver_source *src)
 {
 	assert(src);
 	return src->file_opts;