From ad202eb9b6d04d47fcd7859253959321d08b337f Mon Sep 17 00:00:00 2001
From: Masahiro Nagano <m-nagano@sakura.ad.jp>
Date: Thu, 20 Aug 2026 00:21:05 +0900
Subject: [PATCH 1/2] fix: handle nil message return in internalGo function and
 add tests for error message type

---
 flagrun.go         |  5 +++++
 flagrun_go_test.go | 28 ++++++++++++++++++++++++----
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/flagrun.go b/flagrun.go
index 3b9d314..4cf637e 100644
--- a/flagrun.go
+++ b/flagrun.go
@@ -193,6 +193,11 @@ func internalGo[T any](
 		return "", *c
 	}
 	msg, code := opt.Run(args)
+	// If the message is nil, return an empty string and the code
+	typeOfMsg := reflect.TypeOf(msg)
+	if typeOfMsg == nil || (typeOfMsg.Kind() == reflect.Pointer && reflect.ValueOf(msg).IsNil()) {
+		return "", code
+	}
 	return fmt.Sprintf("%v", msg), code
 }
 
diff --git a/flagrun_go_test.go b/flagrun_go_test.go
index 27ca61f..f82c76a 100644
--- a/flagrun_go_test.go
+++ b/flagrun_go_test.go
@@ -199,7 +199,20 @@ func (r *anyMessageRunner) Run(_ []string) (any, int) {
 	return "Switch is OFF", OK
 }
 
-func TestInternalGoWithAnyMessageType(t *testing.T) {
+type errorMessageRunner struct {
+	Switch bool `short:"s" long:"switch" description:"A boolean switch"`
+}
+
+//nolint:staticcheck // intentional: Runner[error] requires the (error, int) signature.
+func (r *errorMessageRunner) Run(_ []string) (error, int) {
+	if r.Switch {
+		return fmt.Errorf("Switch is %v", r.Switch), CRITICAL
+	}
+	return nil, OK
+}
+
+func runInternalGoMessageTypeTests[T any](t *testing.T, runner Runner[T], wantMsgOff string) {
+	t.Helper()
 	tests := []struct {
 		name     string
 		args     []string
@@ -209,7 +222,7 @@ func TestInternalGoWithAnyMessageType(t *testing.T) {
 		{
 			name:     "Switch is OFF",
 			args:     []string{},
-			wantMsg:  "Switch is OFF",
+			wantMsg:  wantMsgOff,
 			wantCode: OK,
 		},
 		{
@@ -222,13 +235,20 @@ func TestInternalGoWithAnyMessageType(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			o := &anyMessageRunner{}
 			var stdout bytes.Buffer
 			var stderr bytes.Buffer
 			f := buildFlagrun()
-			msg, code := internalGo(f, tt.args, &stdout, &stderr, o) // T is inferred
+			msg, code := internalGo(f, tt.args, &stdout, &stderr, runner)
 			assert.Equal(t, tt.wantMsg, msg, "%s msg", tt.name)
 			assert.Equal(t, code, tt.wantCode, "%s code", tt.name)
 		})
 	}
 }
+
+func TestInternalGoWithAnyMessageType(t *testing.T) {
+	runInternalGoMessageTypeTests(t, &anyMessageRunner{}, "Switch is OFF")
+}
+
+func TestInternalGoWithErrorMessageType(t *testing.T) {
+	runInternalGoMessageTypeTests(t, &errorMessageRunner{}, "")
+}

From 391aa80ef8cceff6ed2f5a08be435bebc79e58bb Mon Sep 17 00:00:00 2001
From: Masahiro Nagano <kazeburo@gmail.com>
Date: Thu, 20 Aug 2026 00:22:51 +0900
Subject: [PATCH 2/2] Update flagrun.go

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
---
 flagrun.go | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/flagrun.go b/flagrun.go
index 4cf637e..8dc8b66 100644
--- a/flagrun.go
+++ b/flagrun.go
@@ -194,10 +194,17 @@ func internalGo[T any](
 	}
 	msg, code := opt.Run(args)
 	// If the message is nil, return an empty string and the code
-	typeOfMsg := reflect.TypeOf(msg)
-	if typeOfMsg == nil || (typeOfMsg.Kind() == reflect.Pointer && reflect.ValueOf(msg).IsNil()) {
+	valueOfMsg := reflect.ValueOf(msg)
+	if !valueOfMsg.IsValid() {
 		return "", code
 	}
+
+	switch valueOfMsg.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
+		if valueOfMsg.IsNil() {
+			return "", code
+		}
+	}
 	return fmt.Sprintf("%v", msg), code
 }