diff --git a/sequencer.c b/sequencer.c
index cc8a361cce..c2cba5ed4b 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -983,7 +983,8 @@ static int run_git_commit(const char *defmsg,
 
 	cmd.git_cmd = 1;
 
-	if (is_rebase_i(opts) && read_env_script(&cmd.env_array)) {
+	if (is_rebase_i(opts) && !(!defmsg && (flags & AMEND_MSG)) &&
+	    read_env_script(&cmd.env_array)) {
 		const char *gpg_opt = gpg_sign_opt_quoted(opts);
 
 		return error(_(staged_changes_advice),
@@ -3815,6 +3816,45 @@ static int do_merge(struct repository *r,
 		goto leave_merge;
 	}
 
+	/*
+	 * If HEAD is not identical to the first parent of the original merge
+	 * commit, we cannot fast-forward.
+	 */
+	can_fast_forward = opts->allow_ff && commit && commit->parents &&
+		oideq(&commit->parents->item->object.oid,
+		      &head_commit->object.oid);
+
+	/*
+	 * If any merge head is different from the original one, we cannot
+	 * fast-forward.
+	 */
+	if (can_fast_forward) {
+		struct commit_list *p = commit->parents->next;
+
+		for (j = to_merge; j && p; j = j->next, p = p->next)
+			if (!oideq(&j->item->object.oid,
+				   &p->item->object.oid)) {
+				can_fast_forward = 0;
+				break;
+			}
+		/*
+		 * If the number of merge heads differs from the original merge
+		 * commit, we cannot fast-forward.
+		 */
+		if (j || p)
+			can_fast_forward = 0;
+	}
+
+	if (can_fast_forward) {
+		rollback_lock_file(&lock);
+		ret = fast_forward_to(r, &commit->object.oid,
+				      &head_commit->object.oid, 0, opts);
+		if (flags & TODO_EDIT_MERGE_MSG)
+			goto fast_forward_edit;
+
+		goto leave_merge;
+	}
+
 	if (commit) {
 		const char *encoding = get_commit_output_encoding();
 		const char *message = logmsg_reencode(commit, NULL, encoding);
@@ -3864,45 +3904,6 @@ static int do_merge(struct repository *r,
 		}
 	}
 
-	/*
-	 * If HEAD is not identical to the first parent of the original merge
-	 * commit, we cannot fast-forward.
-	 */
-	can_fast_forward = opts->allow_ff && commit && commit->parents &&
-		oideq(&commit->parents->item->object.oid,
-		      &head_commit->object.oid);
-
-	/*
-	 * If any merge head is different from the original one, we cannot
-	 * fast-forward.
-	 */
-	if (can_fast_forward) {
-		struct commit_list *p = commit->parents->next;
-
-		for (j = to_merge; j && p; j = j->next, p = p->next)
-			if (!oideq(&j->item->object.oid,
-				   &p->item->object.oid)) {
-				can_fast_forward = 0;
-				break;
-			}
-		/*
-		 * If the number of merge heads differs from the original merge
-		 * commit, we cannot fast-forward.
-		 */
-		if (j || p)
-			can_fast_forward = 0;
-	}
-
-	if (can_fast_forward) {
-		rollback_lock_file(&lock);
-		ret = fast_forward_to(r, &commit->object.oid,
-				      &head_commit->object.oid, 0, opts);
-		if (flags & TODO_EDIT_MERGE_MSG)
-			goto fast_forward_edit;
-
-		goto leave_merge;
-	}
-
 	if (strategy || to_merge->next) {
 		/* Octopus merge */
 		struct child_process cmd = CHILD_PROCESS_INIT;
diff --git a/t/lib-rebase.sh b/t/lib-rebase.sh
index 99d9e7efd2..ec6b9b107d 100644
--- a/t/lib-rebase.sh
+++ b/t/lib-rebase.sh
@@ -173,10 +173,16 @@ set_reword_editor () {
 
 	write_script reword-editor.sh <<-EOF &&
 	# Save the oid of the first reworded commit so we can check rebase
-	# fast-forwards to it
+	# fast-forwards to it. Also check that we do not write .git/MERGE_MSG
+	# when fast-forwarding
 	if ! test -s reword-oid
 	then
-		git rev-parse HEAD >reword-oid
+		git rev-parse HEAD >reword-oid &&
+		if test -f .git/MERGE_MSG
+		then
+			echo 1>&2 "error: .git/MERGE_MSG exists"
+			exit 1
+		fi
 	fi &&
 	# There should be no uncommited changes
 	git diff --exit-code HEAD &&