diff --git a/cleanse.go b/cleanse.go
index a6793d3..e8a2638 100644
--- a/cleanse.go
+++ b/cleanse.go
@@ -1,17 +1,17 @@
 package memoryfs
 
 import (
-    "path/filepath"
-    "strings"
+	"path/filepath"
+	"strings"
 )
 
 func cleanse(path string) string {
-    path = strings.ReplaceAll(path, "/", separator)
-    path = filepath.Clean(path)
-    path = strings.TrimPrefix(path, "."+separator)
-    path = strings.TrimPrefix(path, separator)
-    if path == "." {
-        return ""
-    }
-    return path
+	path = strings.ReplaceAll(path, "/", separator)
+	path = filepath.Clean(path)
+	path = strings.TrimPrefix(path, "."+separator)
+	path = strings.TrimPrefix(path, separator)
+	if path == "." {
+		return ""
+	}
+	return path
 }
diff --git a/dir.go b/dir.go
index e7b421c..e698e38 100644
--- a/dir.go
+++ b/dir.go
@@ -1,6 +1,7 @@
 package memoryfs
 
 import (
+	"fmt"
 	"io"
 	"io/fs"
 	"path/filepath"
@@ -189,13 +190,15 @@ func (d *dir) MkdirAll(path string, perm fs.FileMode) error {
 	}
 
 	d.Lock()
+	if perm&fs.ModeDir == 0 {
+		perm |= fs.ModeDir
+	}
 	if _, ok := d.dirs[parts[0]]; !ok {
 		d.dirs[parts[0]] = &dir{
 			info: fileinfo{
 				name:     parts[0],
 				size:     0x100,
 				modified: time.Now(),
-				isDir:    true,
 				mode:     perm,
 			},
 			dirs:  map[string]*dir{},
@@ -217,6 +220,10 @@ func (d *dir) MkdirAll(path string, perm fs.FileMode) error {
 func (d *dir) WriteFile(path string, data []byte, perm fs.FileMode) error {
 	parts := strings.Split(path, separator)
 
+	if perm&fs.ModeDir != 0 {
+		return fmt.Errorf("invalid perm: %v", perm)
+	}
+
 	if len(parts) == 1 {
 		max := bufferSize
 		if len(data) > max {
@@ -236,7 +243,6 @@ func (d *dir) WriteFile(path string, data []byte, perm fs.FileMode) error {
 					name:     parts[0],
 					size:     int64(len(buffer)),
 					modified: time.Now(),
-					isDir:    false,
 					mode:     perm,
 				},
 				content: buffer,
@@ -304,6 +310,10 @@ func (d *dir) glob(pattern string) ([]string, error) {
 func (d *dir) WriteLazyFile(path string, opener LazyOpener, perm fs.FileMode) error {
 	parts := strings.Split(path, separator)
 
+	if perm&fs.ModeDir != 0 {
+		return fmt.Errorf("invalid perm: %v", perm)
+	}
+
 	if len(parts) == 1 {
 		d.Lock()
 		defer d.Unlock()
@@ -312,7 +322,6 @@ func (d *dir) WriteLazyFile(path string, opener LazyOpener, perm fs.FileMode) er
 				name:     parts[0],
 				size:     0,
 				modified: time.Now(),
-				isDir:    false,
 				mode:     perm,
 			},
 			opener: opener,
diff --git a/file.go b/file.go
index e3606b1..4035c3b 100644
--- a/file.go
+++ b/file.go
@@ -21,7 +21,6 @@ type fileAccess struct {
 	reader io.Reader
 }
 
-
 // LazyOpener provides an io.Reader that can be used to access the content of a file, whatever the actual storage medium.
 // If the LazyOpener returns an io.ReadCloser, it will be closed after each read.
 type LazyOpener func() (io.Reader, error)
diff --git a/fileinfo.go b/fileinfo.go
index dcc5810..cb55dbf 100644
--- a/fileinfo.go
+++ b/fileinfo.go
@@ -9,7 +9,6 @@ type fileinfo struct {
 	name     string
 	size     int64
 	modified time.Time
-	isDir    bool
 	mode     fs.FileMode
 }
 
@@ -46,7 +45,7 @@ func (f fileinfo) ModTime() time.Time {
 
 // IsDir reports whether the entry describes a directory.
 func (f fileinfo) IsDir() bool {
-	return f.isDir
+	return f.Mode().IsDir()
 }
 
 // Sys is the underlying data source of the file (always nil)
diff --git a/fs.go b/fs.go
index 876025f..e7de22c 100644
--- a/fs.go
+++ b/fs.go
@@ -21,8 +21,7 @@ func New() *FS {
 				name:     ".",
 				size:     0x100,
 				modified: time.Now(),
-				isDir:    true,
-				mode:     0o700,
+				mode:     0o0700 | fs.ModeDir,
 			},
 			dirs:  map[string]*dir{},
 			files: map[string]*file{},
diff --git a/fs_test.go b/fs_test.go
index 79940b6..0891ded 100644
--- a/fs_test.go
+++ b/fs_test.go
@@ -39,6 +39,12 @@ func Test_AllOperations(t *testing.T) {
 	require.NoError(t, memfs.WriteFile("files/a/b/c/.secret", []byte("secret file!"), 0o644))
 	require.NoError(t, memfs.WriteFile("files/a/b/c/note.txt", []byte(":)"), 0o644))
 	require.NoError(t, memfs.WriteFile("files/a/middle.txt", []byte(":("), 0o644))
+	require.NoError(t, memfs.MkdirAll("files/xyz", 0o700|fs.ModeDir))
+
+	require.Error(t, memfs.WriteFile("test.txt", []byte("hello world"), 0o644|fs.ModeDir))
+	require.Error(t, memfs.WriteLazyFile("test.txt", func() (io.Reader, error) {
+		return strings.NewReader("hello"), nil
+	}, 0o644|fs.ModeDir))
 
 	t.Run("Open file", func(t *testing.T) {
 		f, err := memfs.Open("test.txt")
@@ -78,6 +84,7 @@ func Test_AllOperations(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, "test.txt", info.Name())
 		assert.Equal(t, fs.FileMode(0o644), info.Mode())
+		assert.Equal(t, false, info.Mode().IsDir())
 		assert.Equal(t, false, info.IsDir())
 		assert.Equal(t, int64(11), info.Size())
 	})
@@ -87,6 +94,7 @@ func Test_AllOperations(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, ".secret", info.Name())
 		assert.Equal(t, fs.FileMode(0o644), info.Mode())
+		assert.Equal(t, false, info.Mode().IsDir())
 		assert.Equal(t, false, info.IsDir())
 		assert.Equal(t, int64(12), info.Size())
 	})
@@ -97,6 +105,16 @@ func Test_AllOperations(t *testing.T) {
 		assert.Nil(t, info)
 	})
 
+	t.Run("Stat directory", func(t *testing.T) {
+		info, err := memfs.Stat("files/xyz")
+		require.NoError(t, err)
+		assert.Equal(t, "xyz", info.Name())
+		assert.Equal(t, fs.FileMode(0o700|fs.ModeDir), info.Mode())
+		assert.Equal(t, true, info.IsDir())
+		assert.Equal(t, true, info.Mode().IsDir())
+		assert.Equal(t, int64(256), info.Size())
+	})
+
 	t.Run("List directory at root", func(t *testing.T) {
 		entries, err := fs.ReadDir(memfs, ".")
 		require.NoError(t, err)
@@ -125,8 +143,9 @@ func Test_AllOperations(t *testing.T) {
 		info, err := memfs.Stat(".")
 		require.NoError(t, err)
 		assert.Equal(t, ".", info.Name())
-		assert.Equal(t, fs.FileMode(0o700), info.Mode())
+		assert.Equal(t, fs.FileMode(0o700|fs.ModeDir), info.Mode())
 		assert.Equal(t, true, info.IsDir())
+		assert.Equal(t, true, info.Mode().IsDir())
 	})
 
 	t.Run("ReadFile", func(t *testing.T) {