diff --git a/vendor/cmd/go/internal/modfetch/codehost/codehost.go b/vendor/cmd/go/internal/modfetch/codehost/codehost.go
index 0e3bb7d..218bb1a 100644
--- a/vendor/cmd/go/internal/modfetch/codehost/codehost.go
+++ b/vendor/cmd/go/internal/modfetch/codehost/codehost.go
@@ -50,6 +50,9 @@
 
 	// ReadFile reads the given file in the file tree corresponding to revision rev.
 	// It should refuse to read more than maxSize bytes.
+	//
+	// If the requested file does not exist it should return an error for which
+	// os.IsNotExist(err) returns true.
 	ReadFile(rev, file string, maxSize int64) (data []byte, err error)
 
 	// ReadZip downloads a zip file for the subdir subdirectory
diff --git a/vendor/cmd/go/internal/modfetch/coderepo.go b/vendor/cmd/go/internal/modfetch/coderepo.go
index bd815e1..703b350 100644
--- a/vendor/cmd/go/internal/modfetch/coderepo.go
+++ b/vendor/cmd/go/internal/modfetch/coderepo.go
@@ -212,7 +212,10 @@
 		gomod1, err1 := r.code.ReadFile(rev, file1, codehost.MaxGoMod)
 		r.codeMu.Unlock()
 		if err1 != nil {
-			return "", "", nil, fmt.Errorf("missing go.mod")
+			if os.IsNotExist(err1) {
+				return "", "", nil, errors.New("missing go.mod")
+			}
+			return "", "", nil, fmt.Errorf("reading go.mod: %v", err1)
 		}
 		return rev, r.codeDir, gomod1, nil
 	}
@@ -230,6 +233,14 @@
 	gomod1, err1 := r.code.ReadFile(rev, file1, codehost.MaxGoMod)
 	gomod2, err2 := r.code.ReadFile(rev, file2, codehost.MaxGoMod)
 	r.codeMu.Unlock()
+
+	if err1 != nil && !os.IsNotExist(err1) {
+		return "", "", nil, fmt.Errorf("reading %s: %v", file1, err1)
+	}
+	if err2 != nil && !os.IsNotExist(err2) {
+		return "", "", nil, fmt.Errorf("reading %s: %v", file2, err2)
+	}
+
 	found1 := err1 == nil && isMajor(gomod1, r.pathMajor)
 	found2 := err2 == nil && isMajor(gomod2, r.pathMajor)
 
@@ -264,7 +275,7 @@
 	data, err = r.code.ReadFile(rev, path.Join(dir, "go.mod"), codehost.MaxGoMod)
 	r.codeMu.Unlock()
 	if err != nil {
-		if e := strings.ToLower(err.Error()); strings.Contains(e, "not found") || strings.Contains(e, "404") { // TODO
+		if os.IsNotExist(err) {
 			return r.legacyGoMod(rev, dir), nil
 		}
 		return nil, err
diff --git a/vendor/cmd/go/internal/modfetch/gitrepo/fetch.go b/vendor/cmd/go/internal/modfetch/gitrepo/fetch.go
index 532d3f3..8a653a8 100644
--- a/vendor/cmd/go/internal/modfetch/gitrepo/fetch.go
+++ b/vendor/cmd/go/internal/modfetch/gitrepo/fetch.go
@@ -279,7 +279,7 @@
 				return &codehost.RevInfo{Version: rev}, out, nil
 			}
 			if bytes.Contains(err.(*codehost.RunError).Stderr, []byte("did not match any files")) {
-				return nil, nil, fmt.Errorf("file not found")
+				return nil, nil, os.ErrNotExist
 			}
 			if bytes.Contains(err.(*codehost.RunError).Stderr, []byte("Operation not supported by protocol")) {
 				r.canArchive = false
@@ -422,7 +422,7 @@
 	}
 	out, err := codehost.Run(r.dir, "git", "cat-file", "blob", info.Name+":"+file)
 	if err != nil {
-		return nil, fmt.Errorf("file not found")
+		return nil, os.ErrNotExist
 	}
 	return out, nil
 }
@@ -441,7 +441,7 @@
 		archive, err = codehost.Run(r.dir, "git", "archive", "--format=zip", "--prefix=prefix/", info.Name, args)
 		if err != nil {
 			if bytes.Contains(err.(*codehost.RunError).Stderr, []byte("did not match any files")) {
-				return nil, "", fmt.Errorf("file not found")
+				return nil, "", os.ErrNotExist
 			}
 			return nil, "", err
 		}
diff --git a/vendor/cmd/go/internal/modfetch/gitrepo/fetch_test.go b/vendor/cmd/go/internal/modfetch/gitrepo/fetch_test.go
index ca93280..b2d2524 100644
--- a/vendor/cmd/go/internal/modfetch/gitrepo/fetch_test.go
+++ b/vendor/cmd/go/internal/modfetch/gitrepo/fetch_test.go
@@ -173,7 +173,7 @@
 		repo: gitrepo1,
 		rev:  "v2.3.4",
 		file: "another.txt",
-		err:  "file not found",
+		err:  os.ErrNotExist.Error(),
 	},
 }