diff --git a/builtin/commit-tree.c b/builtin/commit-tree.c
index b9c331225f..05353c30f2 100644
--- a/builtin/commit-tree.c
+++ b/builtin/commit-tree.c
@@ -98,7 +98,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			die_errno("git commit-tree: failed to read");
 	}
 
-	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1, NULL)) {
+	if (commit_tree(&buffer, tree_sha1, parents, commit_sha1, NULL)) {
 		strbuf_release(&buffer);
 		return 1;
 	}
diff --git a/builtin/commit.c b/builtin/commit.c
index be1ab2e257..3069041b80 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1492,7 +1492,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		append_merge_tag_headers(parents, &tail);
 	}
 
-	if (commit_tree_extended(sb.buf, active_cache_tree->sha1, parents, sha1,
+	if (commit_tree_extended(&sb, active_cache_tree->sha1, parents, sha1,
 				 author_ident.buf, extra)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));
diff --git a/builtin/merge.c b/builtin/merge.c
index 24579409c0..a896165790 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -910,7 +910,8 @@ static int merge_trivial(struct commit *head)
 	parent->next->item = remoteheads->item;
 	parent->next->next = NULL;
 	prepare_to_commit();
-	commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL);
+	if (commit_tree(&merge_msg, result_tree, parent, result_commit, NULL))
+		die(_("failed to write commit object"));
 	finish(head, result_commit, "In-index merge");
 	drop_save();
 	return 0;
@@ -941,7 +942,8 @@ static int finish_automerge(struct commit *head,
 	strbuf_addch(&merge_msg, '\n');
 	prepare_to_commit();
 	free_commit_list(remoteheads);
-	commit_tree(merge_msg.buf, result_tree, parents, result_commit, NULL);
+	if (commit_tree(&merge_msg, result_tree, parents, result_commit, NULL))
+		die(_("failed to write commit object"));
 	strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
 	finish(head, result_commit, buf.buf);
 	strbuf_release(&buf);
diff --git a/builtin/notes.c b/builtin/notes.c
index 667e20a1e1..3644d140ec 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -301,12 +301,12 @@ void commit_notes(struct notes_tree *t, const char *msg)
 		return; /* don't have to commit an unchanged tree */
 
 	/* Prepare commit message and reflog message */
-	strbuf_addstr(&buf, "notes: "); /* commit message starts at index 7 */
 	strbuf_addstr(&buf, msg);
 	if (buf.buf[buf.len - 1] != '\n')
 		strbuf_addch(&buf, '\n'); /* Make sure msg ends with newline */
 
-	create_notes_commit(t, NULL, buf.buf + 7, commit_sha1);
+	create_notes_commit(t, NULL, &buf, commit_sha1);
+	strbuf_insert(&buf, 0, "notes: ", 7); /* commit message starts at index 7 */
 	update_ref(buf.buf, t->ref, commit_sha1, NULL, 0, DIE_ON_ERR);
 
 	strbuf_release(&buf);
diff --git a/commit.c b/commit.c
index b78127403b..44bc96d44d 100644
--- a/commit.c
+++ b/commit.c
@@ -973,7 +973,7 @@ void free_commit_extra_headers(struct commit_extra_header *extra)
 	}
 }
 
-int commit_tree(const char *msg, unsigned char *tree,
+int commit_tree(const struct strbuf *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
 		const char *author)
 {
@@ -991,7 +991,7 @@ 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 char *msg, unsigned char *tree,
+int commit_tree_extended(const struct strbuf *msg, unsigned char *tree,
 			 struct commit_list *parents, unsigned char *ret,
 			 const char *author, struct commit_extra_header *extra)
 {
@@ -1001,6 +1001,9 @@ int commit_tree_extended(const char *msg, unsigned char *tree,
 
 	assert_sha1_type(tree, OBJ_TREE);
 
+	if (memchr(msg->buf, '\0', msg->len))
+		return error("a NUL byte in commit log message not allowed.");
+
 	/* Not having i18n.commitencoding is the same as having utf-8 */
 	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
 
@@ -1037,7 +1040,7 @@ int commit_tree_extended(const char *msg, unsigned char *tree,
 	strbuf_addch(&buffer, '\n');
 
 	/* And add the comment */
-	strbuf_addstr(&buffer, msg);
+	strbuf_addbuf(&buffer, msg);
 
 	/* And check the encoding */
 	if (encoding_is_utf8 && !is_utf8(buffer.buf))
diff --git a/commit.h b/commit.h
index 3745f12099..4df397865a 100644
--- a/commit.h
+++ b/commit.h
@@ -191,11 +191,11 @@ struct commit_extra_header {
 extern void append_merge_tag_headers(struct commit_list *parents,
 				     struct commit_extra_header ***tail);
 
-extern int commit_tree(const char *msg, unsigned char *tree,
+extern int commit_tree(const struct strbuf *msg, unsigned char *tree,
 		       struct commit_list *parents, unsigned char *ret,
 		       const char *author);
 
-extern int commit_tree_extended(const char *msg, unsigned char *tree,
+extern int commit_tree_extended(const struct strbuf *msg, unsigned char *tree,
 				struct commit_list *parents, unsigned char *ret,
 				const char *author,
 				struct commit_extra_header *);
diff --git a/notes-cache.c b/notes-cache.c
index 4c8984ede1..bea013eeae 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -48,6 +48,7 @@ int notes_cache_write(struct notes_cache *c)
 {
 	unsigned char tree_sha1[20];
 	unsigned char commit_sha1[20];
+	struct strbuf msg = STRBUF_INIT;
 
 	if (!c || !c->tree.initialized || !c->tree.ref || !*c->tree.ref)
 		return -1;
@@ -56,7 +57,9 @@ int notes_cache_write(struct notes_cache *c)
 
 	if (write_notes_tree(&c->tree, tree_sha1))
 		return -1;
-	if (commit_tree(c->validity, tree_sha1, NULL, commit_sha1, NULL) < 0)
+	strbuf_attach(&msg, c->validity,
+		      strlen(c->validity), strlen(c->validity) + 1);
+	if (commit_tree(&msg, tree_sha1, NULL, commit_sha1, NULL) < 0)
 		return -1;
 	if (update_ref("update notes cache", c->tree.ref, commit_sha1, NULL,
 		       0, QUIET_ON_ERR) < 0)
diff --git a/notes-merge.c b/notes-merge.c
index 2de27af751..0d38a1b17d 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -530,7 +530,7 @@ static int merge_from_diffs(struct notes_merge_options *o,
 }
 
 void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
-			 const char *msg, unsigned char *result_sha1)
+			 const struct strbuf *msg, unsigned char *result_sha1)
 {
 	unsigned char tree_sha1[20];
 
@@ -668,7 +668,7 @@ int notes_merge(struct notes_merge_options *o,
 		struct commit_list *parents = NULL;
 		commit_list_insert(remote, &parents); /* LIFO order */
 		commit_list_insert(local, &parents);
-		create_notes_commit(local_tree, parents, o->commit_msg.buf,
+		create_notes_commit(local_tree, parents, &o->commit_msg,
 				    result_sha1);
 	}
 
@@ -695,7 +695,8 @@ int notes_merge_commit(struct notes_merge_options *o,
 	struct dir_struct dir;
 	char *path = xstrdup(git_path(NOTES_MERGE_WORKTREE "/"));
 	int path_len = strlen(path), i;
-	const char *msg = strstr(partial_commit->buffer, "\n\n");
+	char *msg = strstr(partial_commit->buffer, "\n\n");
+	struct strbuf sb_msg = STRBUF_INIT;
 
 	if (o->verbosity >= 3)
 		printf("Committing notes in notes merge worktree at %.*s\n",
@@ -733,7 +734,8 @@ int notes_merge_commit(struct notes_merge_options *o,
 				sha1_to_hex(obj_sha1), sha1_to_hex(blob_sha1));
 	}
 
-	create_notes_commit(partial_tree, partial_commit->parents, msg,
+	strbuf_attach(&sb_msg, msg, strlen(msg), strlen(msg) + 1);
+	create_notes_commit(partial_tree, partial_commit->parents, &sb_msg,
 			    result_sha1);
 	if (o->verbosity >= 4)
 		printf("Finalized notes merge commit: %s\n",
diff --git a/notes-merge.h b/notes-merge.h
index 168a6724cd..0c11b173a1 100644
--- a/notes-merge.h
+++ b/notes-merge.h
@@ -37,7 +37,7 @@ void init_notes_merge_options(struct notes_merge_options *o);
  * The resulting commit SHA1 is stored in result_sha1.
  */
 void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
-			 const char *msg, unsigned char *result_sha1);
+			 const struct strbuf *msg, unsigned char *result_sha1);
 
 /*
  * Merge notes from o->remote_ref into o->local_ref
diff --git a/t/t3900-i18n-commit.sh b/t/t3900-i18n-commit.sh
index 1f62c151b0..d48a7c002d 100755
--- a/t/t3900-i18n-commit.sh
+++ b/t/t3900-i18n-commit.sh
@@ -34,6 +34,12 @@ test_expect_success 'no encoding header for base case' '
 	test z = "z$E"
 '
 
+test_expect_failure 'UTF-16 refused because of NULs' '
+	echo UTF-16 >F &&
+	git commit -a -F "$TEST_DIRECTORY"/t3900/UTF-16.txt
+'
+
+
 for H in ISO8859-1 eucJP ISO-2022-JP
 do
 	test_expect_success "$H setup" '