diff --git a/fs.go b/fs.go
index f17d7434..32c70d5c 100644
--- a/fs.go
+++ b/fs.go
@@ -5,6 +5,7 @@
 package gin
 
 import (
+	"fmt"
 	"net/http"
 	"os"
 )
@@ -29,11 +30,12 @@ func Dir(root string, listDirectory bool) http.FileSystem {
 	return &onlyFilesFS{fs}
 }
 
-// Open conforms to http.Filesystem.
+// Open opens a file excluding directories.
+// It conforms to http.Filesystem.
 func (fs onlyFilesFS) Open(name string) (http.File, error) {
 	f, err := fs.fs.Open(name)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to open file: %w", err)
 	}
 	return neuteredReaddirFile{f}, nil
 }
diff --git a/mode.go b/mode.go
index fd26d907..6e69d60b 100644
--- a/mode.go
+++ b/mode.go
@@ -6,6 +6,7 @@ package gin
 
 import (
 	"flag"
+	"fmt"
 	"io"
 	"os"
 
@@ -53,8 +54,16 @@ func init() {
 	SetMode(mode)
 }
 
+func init() {
+	mode := os.Getenv(EnvGinMode)
+	err := SetMode(mode)
+	if err != nil {
+		panic(err)
+	}
+}
+
 // SetMode sets gin mode according to input string.
-func SetMode(value string) {
+func SetMode(value string) error {
 	if value == "" {
 		if flag.Lookup("test.v") != nil {
 			value = TestMode
@@ -71,10 +80,11 @@ func SetMode(value string) {
 	case TestMode:
 		ginMode = testCode
 	default:
-		panic("gin mode unknown: " + value + " (available mode: debug release test)")
+		return fmt.Errorf("gin mode unknown: %s (available mode: debug release test)", value)
 	}
 
 	modeName = value
+	return nil
 }
 
 // DisableBindValidation closes the default validator.
diff --git a/mode_test.go b/mode_test.go
index 2407f463..475b7090 100644
--- a/mode_test.go
+++ b/mode_test.go
@@ -22,30 +22,36 @@ func TestSetMode(t *testing.T) {
 	assert.Equal(t, TestMode, Mode())
 	os.Unsetenv(EnvGinMode)
 
-	SetMode("")
+	err := SetMode("")
+	assert.NoError(t, err)
 	assert.Equal(t, testCode, ginMode)
 	assert.Equal(t, TestMode, Mode())
 
 	tmp := flag.CommandLine
 	flag.CommandLine = flag.NewFlagSet("", flag.ContinueOnError)
-	SetMode("")
+	err = SetMode("")
+	assert.NoError(t, err)
 	assert.Equal(t, debugCode, ginMode)
 	assert.Equal(t, DebugMode, Mode())
 	flag.CommandLine = tmp
 
-	SetMode(DebugMode)
+	err = SetMode(DebugMode)
+	assert.NoError(t, err)
 	assert.Equal(t, debugCode, ginMode)
 	assert.Equal(t, DebugMode, Mode())
 
-	SetMode(ReleaseMode)
+	err = SetMode(ReleaseMode)
+	assert.NoError(t, err)
 	assert.Equal(t, releaseCode, ginMode)
 	assert.Equal(t, ReleaseMode, Mode())
 
-	SetMode(TestMode)
+	err = SetMode(TestMode)
+	assert.NoError(t, err)
 	assert.Equal(t, testCode, ginMode)
 	assert.Equal(t, TestMode, Mode())
 
-	assert.Panics(t, func() { SetMode("unknown") })
+	err = SetMode("unknown")
+	assert.Error(t, err)
 }
 
 func TestDisableBindValidation(t *testing.T) {