diff --git a/gitindex/index.go b/gitindex/index.go
index 7547bb78b..624f099cc 100644
--- a/gitindex/index.go
+++ b/gitindex/index.go
@@ -360,14 +360,17 @@ func IndexGitRepo(opts Options) error {
 	opts.BuildOptions.RepositoryDescription.Source = opts.RepoDir
 	repo, err := git.PlainOpen(opts.RepoDir)
 	if err != nil {
-		return err
+		return fmt.Errorf("git.PlainOpen: %w", err)
 	}
 
 	if err := setTemplatesFromConfig(&opts.BuildOptions.RepositoryDescription, opts.RepoDir); err != nil {
 		log.Printf("setTemplatesFromConfig(%s): %s", opts.RepoDir, err)
 	}
 
-	repoCache := NewRepoCache(opts.RepoCacheDir)
+	var repoCache *RepoCache
+	if opts.Submodules {
+		repoCache = NewRepoCache(opts.RepoCacheDir)
+	}
 
 	// branch => (path, sha1) => repo.
 	repos := map[fileKey]BlobLocation{}
@@ -380,7 +383,7 @@ func IndexGitRepo(opts Options) error {
 
 	branches, err := expandBranches(repo, opts.Branches, opts.BranchPrefix)
 	if err != nil {
-		return err
+		return fmt.Errorf("expandBranches: %w", err)
 	}
 	for _, b := range branches {
 		commit, err := getCommit(repo, opts.BranchPrefix, b)
@@ -389,7 +392,7 @@ func IndexGitRepo(opts Options) error {
 				continue
 			}
 
-			return err
+			return fmt.Errorf("getCommit: %w", err)
 		}
 
 		opts.BuildOptions.RepositoryDescription.Branches = append(opts.BuildOptions.RepositoryDescription.Branches, zoekt.RepositoryBranch{
@@ -403,17 +406,18 @@ func IndexGitRepo(opts Options) error {
 
 		tree, err := commit.Tree()
 		if err != nil {
+			return fmt.Errorf("commit.Tree: %w", err)
 			return err
 		}
 
 		ig, err := newIgnoreMatcher(tree)
 		if err != nil {
-			return err
+			return fmt.Errorf("newIgnoreMatcher: %w", err)
 		}
 
 		files, subVersions, err := TreeToFiles(repo, tree, opts.BuildOptions.RepositoryDescription.URL, repoCache)
 		if err != nil {
-			return err
+			return fmt.Errorf("TreeToFiles: %w", err)
 		}
 		for k, v := range files {
 			if ig.Match(k.Path) {
@@ -458,7 +462,7 @@ func IndexGitRepo(opts Options) error {
 
 	builder, err := build.NewBuilder(opts.BuildOptions)
 	if err != nil {
-		return err
+		return fmt.Errorf("build.NewBuilder: %w", err)
 	}
 	defer builder.Finish()
 
@@ -505,7 +509,7 @@ func IndexGitRepo(opts Options) error {
 				Content:           contents,
 				Branches:          brs,
 			}); err != nil {
-				return err
+				return fmt.Errorf("error adding document with name %s: %w", key.FullPath(), err)
 			}
 		}
 	}
diff --git a/gitindex/submodule.go b/gitindex/submodule.go
index 03c6e2caa..69a012ad5 100644
--- a/gitindex/submodule.go
+++ b/gitindex/submodule.go
@@ -16,6 +16,7 @@ package gitindex
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 
 	"github.com/go-git/go-git/v5/plumbing/format/config"
@@ -37,7 +38,7 @@ func ParseGitModules(content []byte) (map[string]*SubmoduleEntry, error) {
 	// https://stackoverflow.com/a/21375405
 	r, _, err := buf.ReadRune()
 	if err != nil && err != io.EOF {
-		return nil, err
+		return nil, fmt.Errorf("buf.ReadRune: %w", err)
 	}
 	if r != '\uFEFF' {
 		buf.UnreadRune()
@@ -46,7 +47,7 @@ func ParseGitModules(content []byte) (map[string]*SubmoduleEntry, error) {
 	cfg := &config.Config{}
 
 	if err := dec.Decode(cfg); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("error decoding content %s: %w", string(content), err)
 	}
 
 	result := map[string]*SubmoduleEntry{}
diff --git a/gitindex/tree.go b/gitindex/tree.go
index 145cfd69d..27a1319b0 100644
--- a/gitindex/tree.go
+++ b/gitindex/tree.go
@@ -73,15 +73,18 @@ func newRepoWalker(r *git.Repository, repoURL string, repoCache *RepoCache) *rep
 
 // parseModuleMap initializes rw.submodules.
 func (rw *repoWalker) parseModuleMap(t *object.Tree) error {
+	if rw.repoCache == nil {
+		return nil
+	}
 	modEntry, _ := t.File(".gitmodules")
 	if modEntry != nil {
 		c, err := blobContents(&modEntry.Blob)
 		if err != nil {
-			return err
+			return fmt.Errorf("blobContents: %w", err)
 		}
 		mods, err := ParseGitModules(c)
 		if err != nil {
-			return err
+			return fmt.Errorf("ParseGitModules: %w", err)
 		}
 		rw.submodules = map[string]*SubmoduleEntry{}
 		for _, entry := range mods {
@@ -99,7 +102,7 @@ func TreeToFiles(r *git.Repository, t *object.Tree,
 	rw := newRepoWalker(r, repoURL, repoCache)
 
 	if err := rw.parseModuleMap(t); err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("parseModuleMap: %w", err)
 	}
 
 	tw := object.NewTreeWalker(t, true, make(map[plumbing.Hash]bool))
@@ -110,7 +113,7 @@ func TreeToFiles(r *git.Repository, t *object.Tree,
 			break
 		}
 		if err := rw.handleEntry(name, &entry); err != nil {
-			return nil, nil, err
+			return nil, nil, fmt.Errorf("handleEntry: %w", err)
 		}
 	}
 	return rw.tree, rw.subRepoVersions, nil