diff --git a/go/packages/packagestest/export.go b/go/packages/packagestest/export.go
index 5dea613..d792c3c 100644
--- a/go/packages/packagestest/export.go
+++ b/go/packages/packagestest/export.go
@@ -582,7 +582,7 @@
 
 // MustCopyFileTree returns a file set for a module based on a real directory tree.
 // It scans the directory tree anchored at root and adds a Copy writer to the
-// map for every file found.
+// map for every file found. It skips copying files in nested modules.
 // This is to enable the common case in tests where you have a full copy of the
 // package in your testdata.
 // This will panic if there is any kind of error trying to walk the file tree.
@@ -593,6 +593,12 @@
 			return err
 		}
 		if info.IsDir() {
+			// skip nested modules.
+			if path != root {
+				if fi, err := os.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
+					return filepath.SkipDir
+				}
+			}
 			return nil
 		}
 		fragment, err := filepath.Rel(root, path)
diff --git a/go/packages/packagestest/export_test.go b/go/packages/packagestest/export_test.go
index 356dd4b..1172f7c 100644
--- a/go/packages/packagestest/export_test.go
+++ b/go/packages/packagestest/export_test.go
@@ -5,8 +5,11 @@
 package packagestest_test
 
 import (
+	"io/ioutil"
 	"os"
 	"path/filepath"
+	"reflect"
+	"sort"
 	"testing"
 
 	"golang.org/x/tools/go/packages/packagestest"
@@ -180,3 +183,53 @@
 		})
 	}
 }
+
+func TestMustCopyFiles(t *testing.T) {
+	// Create the following test directory structure in a temporary directory.
+	src := map[string]string{
+		// copies all files under the specified directory.
+		"go.mod": "module example.com",
+		"m.go":   "package m",
+		"a/a.go": "package a",
+		// contents from a nested module shouldn't be copied.
+		"nested/go.mod": "module example.com/nested",
+		"nested/m.go":   "package nested",
+		"nested/b/b.go": "package b",
+	}
+
+	tmpDir, err := ioutil.TempDir("", t.Name())
+	if err != nil {
+		t.Fatalf("failed to create a temporary directory: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	for fragment, contents := range src {
+		fullpath := filepath.Join(tmpDir, filepath.FromSlash(fragment))
+		if err := os.MkdirAll(filepath.Dir(fullpath), 0755); err != nil {
+			t.Fatal(err)
+		}
+		if err := ioutil.WriteFile(fullpath, []byte(contents), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	copied := packagestest.MustCopyFileTree(tmpDir)
+	var got []string
+	for fragment := range copied {
+		got = append(got, filepath.ToSlash(fragment))
+	}
+	want := []string{"go.mod", "m.go", "a/a.go"}
+
+	sort.Strings(got)
+	sort.Strings(want)
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("packagestest.MustCopyFileTree = %v, want %v", got, want)
+	}
+
+	// packagestest.Export is happy.
+	exported := packagestest.Export(t, packagestest.Modules, []packagestest.Module{{
+		Name:  "example.com",
+		Files: packagestest.MustCopyFileTree(tmpDir),
+	}})
+	defer exported.Cleanup()
+}