diff --git a/cmd/lsif-go/main.go b/cmd/lsif-go/main.go
index 4d6784a3..951d4e84 100644
--- a/cmd/lsif-go/main.go
+++ b/cmd/lsif-go/main.go
@@ -2,11 +2,13 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/alecthomas/kingpin"
@@ -15,7 +17,7 @@ import (
 	"github.com/sourcegraph/lsif-go/protocol"
 )
 
-const version = "0.4.1"
+const version = "0.5.0"
 const versionString = version + ", protocol version " + protocol.Version
 
 func init() {
@@ -36,6 +38,7 @@ func realMain() error {
 		outFile        string
 		moduleVersion  string
 		repositoryRoot string
+		moduleRoot     string
 		addContents    bool
 	)
 
@@ -47,6 +50,7 @@ func realMain() error {
 	app.Flag("out", "The output file.").Short('o').Default("dump.lsif").StringVar(&outFile)
 	app.Flag("moduleVersion", "Specifies the version of the module defined by this project.").PlaceHolder("version").StringVar(&moduleVersion)
 	app.Flag("repositoryRoot", "Specifies the path of the current repository (inferred automatically via git).").PlaceHolder("root").StringVar(&repositoryRoot)
+	app.Flag("moduleRoot", "Specifies the module root directory relative to the repository").Default(".").StringVar(&moduleRoot)
 	app.Flag("addContents", "Embed file contents into the dump.").Default("false").BoolVar(&addContents)
 
 	_, err := app.Parse(os.Args[1:])
@@ -59,10 +63,10 @@ func realMain() error {
 		if err != nil {
 			return fmt.Errorf("get git root: %v", err)
 		}
-		repositoryRoot = string(toplevel)
+		repositoryRoot = strings.TrimSpace(string(toplevel))
 	}
 
-	projectRoot, err := filepath.Abs(".")
+	projectRoot, err := filepath.Abs(moduleRoot)
 	if err != nil {
 		return fmt.Errorf("get abspath of project root: %v", err)
 	}
@@ -72,6 +76,16 @@ func realMain() error {
 		return fmt.Errorf("get abspath of repository root: %v", err)
 	}
 
+	// Sanity check: Ensure the module root is inside the repository.
+	if !strings.HasPrefix(projectRoot, repositoryRoot) {
+		return errors.New("module root is outside the repository")
+	}
+
+	// Ensure all the dependencies of the specified module are cached.
+	if err := gomod.Download(projectRoot); err != nil {
+		return fmt.Errof("fetching dependencies: %v", err)
+	}
+
 	moduleName, dependencies, err := gomod.ListModules(projectRoot)
 	if err != nil {
 		return err
@@ -96,6 +110,9 @@ func realMain() error {
 		Args:    os.Args[1:],
 	}
 
+	// TODO(@creachadair): With cgo enabled, the indexer cannot handle packages
+	// that include assembly (.s) files. To index such a package you need to
+	// set CGO_ENABLED=0. Consider maybe doing this explicitly, always.
 	indexer := index.NewIndexer(
 		projectRoot,
 		repositoryRoot,
diff --git a/internal/gomod/module.go b/internal/gomod/module.go
index fe5560d4..7a091d10 100644
--- a/internal/gomod/module.go
+++ b/internal/gomod/module.go
@@ -60,7 +60,7 @@ func ListModules(projectRoot string) (string, map[string]string, error) {
 		return "", nil, nil
 	}
 
-	out, err := run(projectRoot, "go", "list", "-m", "all")
+	out, err := run(projectRoot, "go", "list", "-mod=readonly", "-m", "all")
 	if err != nil {
 		return "", nil, fmt.Errorf("failed to list modules: %v", err)
 	}
@@ -81,6 +81,13 @@ func ListModules(projectRoot string) (string, map[string]string, error) {
 	return lines[0], dependencies, nil
 }
 
+// Download fetches all the dependencies of the module in projectRoot.
+func Download(projectRoot string) error {
+	cmd := exec.Command("go", "mod", "download")
+	cmd.Dir = projectRoot
+	return cmd.Run()
+}
+
 // versionPattern matches the form vX.Y.Z.-yyyymmddhhmmss-abcdefabcdef
 var versionPattern = regexp.MustCompile(`^(.*)-(\d{14})-([a-f0-9]{12})$`)