diff --git a/src/os/path.go b/src/os/path.go
index eb996e5fb9..ec6a7938b2 100644
--- a/src/os/path.go
+++ b/src/os/path.go
@@ -39,8 +39,10 @@ func MkdirAll(path string, perm FileMode) error {
 	}
 
 	if j > 1 {
-		// Create parent
-		err = MkdirAll(path[0:j-1], perm)
+		// Create parent.
+		// Pass trailing path separator to MkdirAll, so our
+		// algorithm works for paths, like \\?\c:\
+		err = MkdirAll(path[0:j], perm)
 		if err != nil {
 			return err
 		}
diff --git a/src/os/path_windows_test.go b/src/os/path_windows_test.go
index cce0bdd522..00a3e63bf3 100644
--- a/src/os/path_windows_test.go
+++ b/src/os/path_windows_test.go
@@ -5,8 +5,10 @@
 package os_test
 
 import (
+	"io/ioutil"
 	"os"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -44,3 +46,31 @@ func TestFixLongPath(t *testing.T) {
 		}
 	}
 }
+
+func TestMkdirAllExtendedLength(t *testing.T) {
+	tmpDir, err := ioutil.TempDir("", "TestMkdirAllExtendedLength")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	const prefix = `\\?\`
+	if len(tmpDir) < 4 || tmpDir[:4] != prefix {
+		fullPath, err := syscall.FullPath(tmpDir)
+		if err != nil {
+			t.Fatalf("FullPath(%q) fails: %v", tmpDir, err)
+		}
+		tmpDir = prefix + fullPath
+	}
+	path := tmpDir + `\dir\`
+	err = os.MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll(%q) failed: %v", path, err)
+	}
+
+	path = path + `.\dir2`
+	err = os.MkdirAll(path, 0777)
+	if err == nil {
+		t.Fatalf("MkdirAll(%q) should have failed, but did not", path)
+	}
+}