diff --git a/sequencer.c b/sequencer.c
index a7e6db4f78..1c96a75b1e 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2479,12 +2479,11 @@ static int is_command(enum todo_command command, const char **bol)
 {
 	const char *str = todo_command_info[command].str;
 	const char nick = todo_command_info[command].c;
-	const char *p = *bol + 1;
+	const char *p = *bol;
 
-	return skip_prefix(*bol, str, bol) ||
-		((nick && **bol == nick) &&
-		 (*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r' || !*p) &&
-		 (*bol = p));
+	return (skip_prefix(p, str, &p) || (nick && *p++ == nick)) &&
+		(*p == ' ' || *p == '\t' || *p == '\n' || *p == '\r' || !*p) &&
+		(*bol = p);
 }
 
 static int check_label_or_ref_arg(enum todo_command command, const char *arg)
@@ -2541,7 +2540,8 @@ static int parse_insn_line(struct repository *r, struct todo_item *item,
 			break;
 		}
 	if (i >= TODO_COMMENT)
-		return -1;
+		return error(_("invalid command '%.*s'"),
+			     (int)strcspn(bol, " \t\r\n"), bol);
 
 	/* Eat up extra spaces/ tabs before object name */
 	padding = strspn(bol, " \t");
@@ -2579,12 +2579,10 @@ static int parse_insn_line(struct repository *r, struct todo_item *item,
 	}
 
 	if (item->command == TODO_FIXUP) {
-		if (skip_prefix(bol, "-C", &bol) &&
-		   (*bol == ' ' || *bol == '\t')) {
+		if (skip_prefix(bol, "-C", &bol)) {
 			bol += strspn(bol, " \t");
 			item->flags |= TODO_REPLACE_FIXUP_MSG;
-		} else if (skip_prefix(bol, "-c", &bol) &&
-				  (*bol == ' ' || *bol == '\t')) {
+		} else if (skip_prefix(bol, "-c", &bol)) {
 			bol += strspn(bol, " \t");
 			item->flags |= TODO_EDIT_FIXUP_MSG;
 		}
diff --git a/t/lib-rebase.sh b/t/lib-rebase.sh
index b57541356b..7ca5b918f0 100644
--- a/t/lib-rebase.sh
+++ b/t/lib-rebase.sh
@@ -60,7 +60,7 @@ set_fake_editor () {
 		">")
 			echo >> "$1";;
 		bad)
-			action="badcmd";;
+			action="pickled";;
 		fakesha)
 			test \& != "$action" || action=pick
 			echo "$action XXXXXXX False commit" >> "$1"
@@ -211,6 +211,9 @@ check_reworded_commits () {
 # usage: set_replace_editor <file>
 #
 # Replace the todo file with the exact contents of the given file.
+# N.B. sets GIT_SEQUENCE_EDITOR rather than EDITOR so it can be
+# combined with set_fake_editor to reword commits and replace the
+# todo list
 set_replace_editor () {
 	cat >script <<-\EOF &&
 	cat FILENAME >"$1"
@@ -219,6 +222,7 @@ set_replace_editor () {
 	cat "$1"
 	EOF
 
-	sed -e "s/FILENAME/$1/g" <script | write_script fake-editor.sh &&
-	test_set_editor "$(pwd)/fake-editor.sh"
+	sed -e "s/FILENAME/$1/g" script |
+		write_script fake-sequence-editor.sh &&
+	test_set_sequence_editor "$(pwd)/fake-sequence-editor.sh"
 }
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index efeb74ad50..ff0afad63e 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1449,14 +1449,15 @@ test_expect_success 'rebase --edit-todo respects rebase.missingCommitsCheck = ig
 
 test_expect_success 'rebase --edit-todo respects rebase.missingCommitsCheck = warn' '
 	cat >expect <<-EOF &&
-	error: invalid line 1: badcmd $(git rev-list --pretty=oneline --abbrev-commit -1 primary~4)
+	error: invalid command '\''pickled'\''
+	error: invalid line 1: pickled $(git rev-list --pretty=oneline --abbrev-commit -1 primary~4)
 	Warning: some commits may have been dropped accidentally.
 	Dropped commits (newer to older):
 	 - $(git rev-list --pretty=oneline --abbrev-commit -1 primary)
 	 - $(git rev-list --pretty=oneline --abbrev-commit -1 primary~4)
 	To avoid this message, use "drop" to explicitly remove a commit.
 	EOF
-	head -n4 expect >expect.2 &&
+	head -n5 expect >expect.2 &&
 	tail -n1 expect >>expect.2 &&
 	tail -n4 expect.2 >expect.3 &&
 	test_config rebase.missingCommitsCheck warn &&
@@ -1467,7 +1468,7 @@ test_expect_success 'rebase --edit-todo respects rebase.missingCommitsCheck = wa
 			git rebase -i --root &&
 		cp .git/rebase-merge/git-rebase-todo.backup orig &&
 		FAKE_LINES="2 3 4" git rebase --edit-todo 2>actual.2 &&
-		head -n6 actual.2 >actual &&
+		head -n7 actual.2 >actual &&
 		test_cmp expect actual &&
 		cp orig .git/rebase-merge/git-rebase-todo &&
 		FAKE_LINES="1 2 3 4" git rebase --edit-todo 2>actual.2 &&
@@ -1483,7 +1484,8 @@ test_expect_success 'rebase --edit-todo respects rebase.missingCommitsCheck = wa
 
 test_expect_success 'rebase --edit-todo respects rebase.missingCommitsCheck = error' '
 	cat >expect <<-EOF &&
-	error: invalid line 1: badcmd $(git rev-list --pretty=oneline --abbrev-commit -1 primary~4)
+	error: invalid command '\''pickled'\''
+	error: invalid line 1: pickled $(git rev-list --pretty=oneline --abbrev-commit -1 primary~4)
 	Warning: some commits may have been dropped accidentally.
 	Dropped commits (newer to older):
 	 - $(git rev-list --pretty=oneline --abbrev-commit -1 primary)
@@ -1583,7 +1585,7 @@ test_expect_success 'static check of bad command' '
 		set_fake_editor &&
 		test_must_fail env FAKE_LINES="1 2 3 bad 4 5" \
 		git rebase -i --root 2>actual &&
-		test_i18ngrep "badcmd $(git rev-list --oneline -1 primary~1)" \
+		test_i18ngrep "pickled $(git rev-list --oneline -1 primary~1)" \
 				actual &&
 		test_i18ngrep "You can fix this with .git rebase --edit-todo.." \
 				actual &&
diff --git a/t/t3437-rebase-fixup-options.sh b/t/t3437-rebase-fixup-options.sh
index 274699dadb..dd3b301fa7 100755
--- a/t/t3437-rebase-fixup-options.sh
+++ b/t/t3437-rebase-fixup-options.sh
@@ -51,6 +51,7 @@ test_expect_success 'setup' '
 	body
 	EOF
 
+	test_commit initial &&
 	test_commit A A &&
 	test_commit B B &&
 	get_author HEAD >expected-author &&
@@ -209,4 +210,29 @@ test_expect_success 'fixup -C works upon --autosquash with amend!' '
 		actual-squash-message
 '
 
+test_expect_success 'fixup -[Cc]<commit> works' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	cat >todo <<-\EOF &&
+	pick A
+	fixup -CA1
+	pick B
+	fixup -cA2
+	EOF
+	(
+		set_replace_editor todo &&
+		FAKE_COMMIT_MESSAGE="edited and fixed up" \
+			git rebase -i initial initial
+	) &&
+	git log --pretty=format:%B initial.. >actual &&
+	cat >expect <<-EOF &&
+	edited and fixed up
+	$EMPTY
+	new subject
+	$EMPTY
+	new
+	body
+	EOF
+	test_cmp expect actual
+'
+
 test_done
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 58cfd2f1fd..999d46fafe 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -32,6 +32,14 @@ test_set_editor () {
 	export EDITOR
 }
 
+# Like test_set_editor but sets GIT_SEQUENCE_EDITOR instead of EDITOR
+test_set_sequence_editor () {
+	FAKE_SEQUENCE_EDITOR="$1"
+	export FAKE_SEQUENCE_EDITOR
+	GIT_SEQUENCE_EDITOR='"$FAKE_SEQUENCE_EDITOR"'
+	export GIT_SEQUENCE_EDITOR
+}
+
 test_decode_color () {
 	awk '
 		function name(n) {