diff --git a/internal/imports/sourcex_test.go b/internal/imports/sourcex_test.go
index e8a4d537f8f..0a2327ca300 100644
--- a/internal/imports/sourcex_test.go
+++ b/internal/imports/sourcex_test.go
@@ -93,7 +93,7 @@ type dirs struct {
 func testDirs(t *testing.T) dirs {
 	t.Helper()
 	dir := t.TempDir()
-	modindex.IndexDir = func() (string, error) { return dir, nil }
+	modindex.IndexDir = dir
 	x := dirs{
 		tmpdir:   dir,
 		cachedir: filepath.Join(dir, "pkg", "mod"),
diff --git a/internal/modindex/dir_test.go b/internal/modindex/dir_test.go
index 6e76f825116..e0919e4c4bf 100644
--- a/internal/modindex/dir_test.go
+++ b/internal/modindex/dir_test.go
@@ -47,10 +47,8 @@ var idtests = []id{
 }
 
 func testModCache(t *testing.T) string {
-	t.Helper()
-	dir := t.TempDir()
-	IndexDir = func() (string, error) { return dir, nil }
-	return dir
+	IndexDir = t.TempDir()
+	return IndexDir
 }
 
 // add a trivial package to the test module cache
@@ -211,11 +209,7 @@ func TestMissingCachedir(t *testing.T) {
 	if err := Create(dir); err != nil {
 		t.Fatal(err)
 	}
-	ixd, err := IndexDir()
-	if err != nil {
-		t.Fatal(err)
-	}
-	des, err := os.ReadDir(ixd)
+	des, err := os.ReadDir(IndexDir)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -232,11 +226,7 @@ func TestMissingIndex(t *testing.T) {
 	} else if !ok {
 		t.Error("Update returned !ok")
 	}
-	ixd, err := IndexDir()
-	if err != nil {
-		t.Fatal(err)
-	}
-	des, err := os.ReadDir(ixd)
+	des, err := os.ReadDir(IndexDir)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/internal/modindex/gomodindex/cmd.go b/internal/modindex/gomodindex/cmd.go
index 06314826422..4fc0caf400e 100644
--- a/internal/modindex/gomodindex/cmd.go
+++ b/internal/modindex/gomodindex/cmd.go
@@ -93,10 +93,7 @@ func query(dir string) {
 	panic("implement")
 }
 func clean(_ string) {
-	des, err := modindex.IndexDir()
-	if err != nil {
-		log.Fatal(err)
-	}
+	des := modindex.IndexDir
 	// look at the files starting with 'index'
 	// the current ones of each version are pointed to by
 	// index-name-%d files. Any others more than an hour old
diff --git a/internal/modindex/index.go b/internal/modindex/index.go
index 9d1d137db3a..9665356c01b 100644
--- a/internal/modindex/index.go
+++ b/internal/modindex/index.go
@@ -17,6 +17,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"testing"
 	"time"
 )
 
@@ -85,6 +86,28 @@ type Entry struct {
 	Names []string // exported names and information
 }
 
+// IndexDir is where the module index is stored.
+var IndexDir string
+
+// Set IndexDir
+func init() {
+	var dir string
+	var err error
+	if testing.Testing() {
+		dir = os.TempDir()
+	} else {
+		dir, err = os.UserCacheDir()
+		// shouldn't happen, but TempDir is better than
+		// creating ./go/imports
+		if err != nil {
+			dir = os.TempDir()
+		}
+	}
+	dir = filepath.Join(dir, "go", "imports")
+	os.MkdirAll(dir, 0777)
+	IndexDir = dir
+}
+
 // ReadIndex reads the latest version of the on-disk index
 // for the cache directory cd.
 // It returns (nil, nil) if there is no index, but returns
@@ -95,10 +118,7 @@ func ReadIndex(cachedir string) (*Index, error) {
 		return nil, err
 	}
 	cd := Abspath(cachedir)
-	dir, err := IndexDir()
-	if err != nil {
-		return nil, err
-	}
+	dir := IndexDir
 	base := indexNameBase(cd)
 	iname := filepath.Join(dir, base)
 	buf, err := os.ReadFile(iname)
@@ -185,12 +205,8 @@ func readIndexFrom(cd Abspath, bx io.Reader) (*Index, error) {
 
 // write the index as a text file
 func writeIndex(cachedir Abspath, ix *Index) error {
-	dir, err := IndexDir()
-	if err != nil {
-		return err
-	}
 	ipat := fmt.Sprintf("index-%d-*", CurrentVersion)
-	fd, err := os.CreateTemp(dir, ipat)
+	fd, err := os.CreateTemp(IndexDir, ipat)
 	if err != nil {
 		return err // can this happen?
 	}
@@ -201,7 +217,7 @@ func writeIndex(cachedir Abspath, ix *Index) error {
 	content := fd.Name()
 	content = filepath.Base(content)
 	base := indexNameBase(cachedir)
-	nm := filepath.Join(dir, base)
+	nm := filepath.Join(IndexDir, base)
 	err = os.WriteFile(nm, []byte(content), 0666)
 	if err != nil {
 		return err
@@ -241,18 +257,6 @@ func writeIndexToFile(x *Index, fd *os.File) error {
 	return nil
 }
 
-// tests can override this
-var IndexDir = indexDir
-
-// indexDir computes the directory containing the index
-func indexDir() (string, error) {
-	dir, err := os.UserCacheDir()
-	if err != nil {
-		return "", fmt.Errorf("cannot open UserCacheDir, %w", err)
-	}
-	return filepath.Join(dir, "go", "imports"), nil
-}
-
 // return the base name of the file containing the name of the current index
 func indexNameBase(cachedir Abspath) string {
 	// crc64 is a way to convert path names into 16 hex digits.