diff --git a/merge-ort.c b/merge-ort.c
index 99dcee2db8a..f654296220e 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -3571,15 +3571,15 @@ static int tree_entry_order(const void *a_, const void *b_)
 				 b->string, strlen(b->string), bmi->result.mode);
 }
 
-static void write_tree(struct object_id *result_oid,
-		       struct string_list *versions,
-		       unsigned int offset,
-		       size_t hash_size)
+static int write_tree(struct object_id *result_oid,
+		      struct string_list *versions,
+		      unsigned int offset,
+		      size_t hash_size)
 {
 	size_t maxlen = 0, extra;
 	unsigned int nr;
 	struct strbuf buf = STRBUF_INIT;
-	int i;
+	int i, ret;
 
 	assert(offset <= versions->nr);
 	nr = versions->nr - offset;
@@ -3605,8 +3605,9 @@ static void write_tree(struct object_id *result_oid,
 	}
 
 	/* Write this object file out, and record in result_oid */
-	write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid);
+	ret = write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid);
 	strbuf_release(&buf);
+	return ret;
 }
 
 static void record_entry_for_tree(struct directory_versions *dir_metadata,
@@ -3625,13 +3626,13 @@ static void record_entry_for_tree(struct directory_versions *dir_metadata,
 			   basename)->util = &mi->result;
 }
 
-static void write_completed_directory(struct merge_options *opt,
-				      const char *new_directory_name,
-				      struct directory_versions *info)
+static int write_completed_directory(struct merge_options *opt,
+				     const char *new_directory_name,
+				     struct directory_versions *info)
 {
 	const char *prev_dir;
 	struct merged_info *dir_info = NULL;
-	unsigned int offset;
+	unsigned int offset, ret = 0;
 
 	/*
 	 * Some explanation of info->versions and info->offsets...
@@ -3717,7 +3718,7 @@ static void write_completed_directory(struct merge_options *opt,
 	 * strcmp here.)
 	 */
 	if (new_directory_name == info->last_directory)
-		return;
+		return 0;
 
 	/*
 	 * If we are just starting (last_directory is NULL), or last_directory
@@ -3739,7 +3740,7 @@ static void write_completed_directory(struct merge_options *opt,
 		 */
 		string_list_append(&info->offsets,
 				   info->last_directory)->util = (void*)offset;
-		return;
+		return 0;
 	}
 
 	/*
@@ -3769,8 +3770,9 @@ static void write_completed_directory(struct merge_options *opt,
 		 */
 		dir_info->is_null = 0;
 		dir_info->result.mode = S_IFDIR;
-		write_tree(&dir_info->result.oid, &info->versions, offset,
-			   opt->repo->hash_algo->rawsz);
+		if (write_tree(&dir_info->result.oid, &info->versions, offset,
+			       opt->repo->hash_algo->rawsz) < 0)
+			ret = -1;
 	}
 
 	/*
@@ -3798,6 +3800,8 @@ static void write_completed_directory(struct merge_options *opt,
 	/* And, of course, we need to update last_directory to match. */
 	info->last_directory = new_directory_name;
 	info->last_directory_len = strlen(info->last_directory);
+
+	return ret;
 }
 
 /* Per entry merge function */
@@ -4214,8 +4218,8 @@ static void prefetch_for_content_merges(struct merge_options *opt,
 	oid_array_clear(&to_fetch);
 }
 
-static void process_entries(struct merge_options *opt,
-			    struct object_id *result_oid)
+static int process_entries(struct merge_options *opt,
+			   struct object_id *result_oid)
 {
 	struct hashmap_iter iter;
 	struct strmap_entry *e;
@@ -4224,11 +4228,12 @@ static void process_entries(struct merge_options *opt,
 	struct directory_versions dir_metadata = { STRING_LIST_INIT_NODUP,
 						   STRING_LIST_INIT_NODUP,
 						   NULL, 0 };
+	int ret;
 
 	trace2_region_enter("merge", "process_entries setup", opt->repo);
 	if (strmap_empty(&opt->priv->paths)) {
 		oidcpy(result_oid, opt->repo->hash_algo->empty_tree);
-		return;
+		return 0;
 	}
 
 	/* Hack to pre-allocate plist to the desired size */
@@ -4270,8 +4275,11 @@ static void process_entries(struct merge_options *opt,
 		 */
 		struct merged_info *mi = entry->util;
 
-		write_completed_directory(opt, mi->directory_name,
-					  &dir_metadata);
+		if (write_completed_directory(opt, mi->directory_name,
+					      &dir_metadata) < 0) {
+			ret = -1;
+			goto cleanup;
+		}
 		if (mi->clean)
 			record_entry_for_tree(&dir_metadata, path, mi);
 		else {
@@ -4291,12 +4299,15 @@ static void process_entries(struct merge_options *opt,
 		fflush(stdout);
 		BUG("dir_metadata accounting completely off; shouldn't happen");
 	}
-	write_tree(result_oid, &dir_metadata.versions, 0,
-		   opt->repo->hash_algo->rawsz);
+	ret = write_tree(result_oid, &dir_metadata.versions, 0,
+			 opt->repo->hash_algo->rawsz);
+cleanup:
 	string_list_clear(&plist, 0);
 	string_list_clear(&dir_metadata.versions, 0);
 	string_list_clear(&dir_metadata.offsets, 0);
 	trace2_region_leave("merge", "process_entries cleanup", opt->repo);
+
+	return ret;
 }
 
 /*** Function Grouping: functions related to merge_switch_to_result() ***/
@@ -4928,15 +4939,18 @@ redo:
 	}
 
 	trace2_region_enter("merge", "process_entries", opt->repo);
-	process_entries(opt, &working_tree_oid);
+	if (process_entries(opt, &working_tree_oid) < 0)
+		result->clean = -1;
 	trace2_region_leave("merge", "process_entries", opt->repo);
 
 	/* Set return values */
 	result->path_messages = &opt->priv->conflicts;
 
-	result->tree = parse_tree_indirect(&working_tree_oid);
-	/* existence of conflicted entries implies unclean */
-	result->clean &= strmap_empty(&opt->priv->conflicted);
+	if (result->clean >= 0) {
+		result->tree = parse_tree_indirect(&working_tree_oid);
+		/* existence of conflicted entries implies unclean */
+		result->clean &= strmap_empty(&opt->priv->conflicted);
+	}
 	if (!opt->priv->call_depth) {
 		result->priv = opt->priv;
 		result->_properly_initialized = RESULT_INITIALIZED;
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index 28ca5c38bb5..013b77144bd 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -810,4 +810,13 @@ test_expect_success 'can override merge of unrelated histories' '
 	test_cmp expect actual
 '
 
+test_expect_success SANITY 'merge-ort fails gracefully in a read-only repository' '
+	git init --bare read-only &&
+	git push read-only side1 side2 side3 &&
+	test_when_finished "chmod -R u+w read-only" &&
+	chmod -R a-w read-only &&
+	test_must_fail git -C read-only merge-tree side1 side3 &&
+	test_must_fail git -C read-only merge-tree side1 side2
+'
+
 test_done
