diff --git a/vendor/cmd/go/internal/modfetch/convert.go b/vendor/cmd/go/internal/modfetch/convert.go
index 8b06b39..7166bfc 100644
--- a/vendor/cmd/go/internal/modfetch/convert.go
+++ b/vendor/cmd/go/internal/modfetch/convert.go
@@ -6,6 +6,7 @@
 
 import (
 	"fmt"
+	"os"
 	"sort"
 	"strings"
 
@@ -50,10 +51,12 @@
 
 		repo, err := Lookup(r.Path)
 		if err != nil {
+			fmt.Fprintf(os.Stderr, "vgo: lookup %s: %v", r.Path, err)
 			continue
 		}
 		info, err := repo.Stat(r.Version)
 		if err != nil {
+			fmt.Fprintf(os.Stderr, "vgo: stat %s@%s: %v", r.Path, r.Version, err)
 			continue
 		}
 		path := repo.ModulePath()
diff --git a/vendor/cmd/go/internal/modfile/rule.go b/vendor/cmd/go/internal/modfile/rule.go
index 640c287..9c50102 100644
--- a/vendor/cmd/go/internal/modfile/rule.go
+++ b/vendor/cmd/go/internal/modfile/rule.go
@@ -61,6 +61,21 @@
 	})
 }
 
+func (f *File) AddComment(text string) {
+	if f.Syntax == nil {
+		f.Syntax = new(FileSyntax)
+	}
+	f.Syntax.Stmt = append(f.Syntax.Stmt, &CommentBlock{
+		Comments: Comments{
+			Before: []Comment{
+				{
+					Token: text,
+				},
+			},
+		},
+	})
+}
+
 type VersionFixer func(path, version string) (string, error)
 
 func Parse(file string, data []byte, fix VersionFixer) (*File, error) {
diff --git a/vendor/cmd/go/internal/vgo/init.go b/vendor/cmd/go/internal/vgo/init.go
index 13b5a68..0c595d1 100644
--- a/vendor/cmd/go/internal/vgo/init.go
+++ b/vendor/cmd/go/internal/vgo/init.go
@@ -142,6 +142,12 @@
 		f.AddModuleStmt(path)
 	}
 
+	if len(f.Syntax.Stmt) == 1 && f.Module != nil {
+		// Entire file is just a module statement.
+		// Populate require if possible.
+		legacyModInit()
+	}
+
 	excluded = make(map[module.Version]bool)
 	for _, x := range f.Exclude {
 		excluded[x.Mod] = true
@@ -155,15 +161,17 @@
 }
 
 func legacyModInit() {
-	path, err := FindModulePath(ModRoot)
-	if err != nil {
-		base.Fatalf("vgo: %v", err)
+	if modFile == nil {
+		path, err := FindModulePath(ModRoot)
+		if err != nil {
+			base.Fatalf("vgo: %v", err)
+		}
+		fmt.Fprintf(os.Stderr, "vgo: creating new go.mod: module %s\n", path)
+		modFile = new(modfile.File)
+		modFile.AddModuleStmt(path)
 	}
-	fmt.Fprintf(os.Stderr, "vgo: module %s\n", path)
-	modFile = new(modfile.File)
-	modFile.AddModuleStmt(path)
-	Target = module.Version{Path: path}
 
+	Target = modFile.Module.Mod
 	for _, name := range altConfigs {
 		cfg := filepath.Join(ModRoot, name)
 		data, err := ioutil.ReadFile(cfg)
@@ -172,14 +180,17 @@
 			if convert == nil {
 				return
 			}
+			fmt.Fprintf(os.Stderr, "vgo: copying requirements from %s\n", cfg)
 			if err := modfetch.ConvertLegacyConfig(modFile, cfg, data); err != nil {
 				base.Fatalf("vgo: %v", err)
 			}
+			if len(modFile.Syntax.Stmt) == 1 {
+				// Add comment to prevent vgo from re-converting every time it runs.
+				modFile.AddComment("// vgo: no requirements found in " + name)
+			}
 			return
 		}
 	}
-
-	base.Fatalf("vgo: internal error: cannot find legacy config file (it was here a minute ago!)")
 }
 
 var altConfigs = []string{
diff --git a/vendor/cmd/go/vgo_test.go b/vendor/cmd/go/vgo_test.go
index b1c32dd..70d1086 100644
--- a/vendor/cmd/go/vgo_test.go
+++ b/vendor/cmd/go/vgo_test.go
@@ -7,6 +7,7 @@
 import (
 	"cmd/go/internal/modconv"
 	"cmd/go/internal/vgo"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"runtime"
@@ -135,3 +136,44 @@
 	tg.run("-vgo", "list", "-f={{.GoFiles}}", "-tags", "tag1 tag2")
 	tg.grepStdout(`\[x.go y.go\]`, "Go source files for tag1 and tag2")
 }
+
+func TestFillGoMod(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+
+	tg.setenv("HOME", tg.path("."))
+	tg.must(os.MkdirAll(tg.path("x"), 0777))
+	tg.must(ioutil.WriteFile(tg.path("x/x.go"), []byte(`
+		package x
+	`), 0666))
+
+	tg.must(ioutil.WriteFile(tg.path("x/go.mod"), []byte(`
+		module x
+	`), 0666))
+	tg.must(ioutil.WriteFile(tg.path("x/Gopkg.lock"), []byte(`
+[[projects]]
+  name = "rsc.io/sampler"
+  version = "v1.0.0"
+	`), 0666))
+
+	tg.cd(tg.path("x"))
+	tg.run("-vgo", "build", "-v")
+	tg.grepStderr("copying requirements from .*Gopkg.lock", "did not copy Gopkg.lock")
+	tg.run("-vgo", "list", "-m")
+	tg.grepStderrNot("copying requirements from .*Gopkg.lock", "should not copy Gopkg.lock again")
+	tg.grepStdout("rsc.io/sampler.*v1.0.0", "did not copy Gopkg.lock")
+
+	tg.must(ioutil.WriteFile(tg.path("x/go.mod"), []byte(`
+		module x
+	`), 0666))
+	tg.must(ioutil.WriteFile(tg.path("x/Gopkg.lock"), []byte(`
+	`), 0666))
+
+	tg.run("-vgo", "list")
+	tg.grepStderr("copying requirements from .*Gopkg.lock", "did not copy Gopkg.lock")
+	tg.run("-vgo", "list")
+	tg.grepStderrNot("copying requirements from .*Gopkg.lock", "should not copy Gopkg.lock again")
+
+}