diff --git a/gopmod/classfile.go b/gopmod/classfile.go
index 0ffed46..b132bf2 100644
--- a/gopmod/classfile.go
+++ b/gopmod/classfile.go
@@ -17,8 +17,7 @@
 package gopmod
 
 import (
-	"syscall"
-
+	"github.com/goplus/mod"
 	"github.com/goplus/mod/modcache"
 	"github.com/goplus/mod/modfetch"
 	"github.com/goplus/mod/modfile"
@@ -46,9 +45,17 @@ var (
 )
 
 var (
+	ErrNotFound        = mod.ErrNotFound
 	ErrNotClassFileMod = errors.New("not a classfile module")
 )
 
+// IsNotFound returns a boolean indicating whether the error is known to
+// report that a module or package does not exist. It is satisfied by
+// ErrNotFound.
+func IsNotFound(err error) bool {
+	return errors.Err(err) == ErrNotFound
+}
+
 // -----------------------------------------------------------------------------
 
 // ClassKind checks a fname is a known classfile or not.
@@ -98,10 +105,10 @@ func (p *Module) ImportClasses(importClass ...func(c *Project)) (err error) {
 func (p *Module) importMod(modPath string, imcls func(c *Project)) (err error) {
 	mod, ok := p.LookupDepMod(modPath)
 	if !ok {
-		return syscall.ENOENT
+		return ErrNotFound
 	}
 	err = p.importClassFrom(mod, imcls)
-	if errors.Err(err) != syscall.ENOENT {
+	if !IsNotFound(err) {
 		return
 	}
 	mod, err = modfetch.Get(mod.String())
diff --git a/gopmod/gopmod_test.go b/gopmod/gopmod_test.go
index e41670e..ec3dc82 100644
--- a/gopmod/gopmod_test.go
+++ b/gopmod/gopmod_test.go
@@ -85,6 +85,11 @@ func TestClassfile(t *testing.T) {
 	if !mod.IsClass("_yap.gox") {
 		t.Fatal("mod.IsClass _yap.gox: not ok?")
 	}
+
+	modVer = module.Version{Path: "github.com/unknown-repo/x", Version: "v0.5.0"}
+	if _, err = LoadMod(modVer); !IsNotFound(err) {
+		log.Fatal("LoadMod github.com/unknown-repo/x:", err)
+	}
 }
 
 func TestClassfile2(t *testing.T) {
diff --git a/gopmod/module.go b/gopmod/module.go
index 27f2b3a..c32a73c 100644
--- a/gopmod/module.go
+++ b/gopmod/module.go
@@ -22,7 +22,6 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
-	"syscall"
 
 	"github.com/goplus/mod/modcache"
 	"github.com/goplus/mod/modfetch"
@@ -173,7 +172,7 @@ func LoadFrom(gomod, gopmod string) (*Module, error) {
 // If we only want to load a Go modfile, pass env parameter as nil.
 func LoadMod(mod module.Version) (p *Module, err error) {
 	p, err = loadModFrom(mod)
-	if errors.Err(err) != syscall.ENOENT {
+	if !IsNotFound(err) {
 		return
 	}
 	mod, err = modfetch.Get(mod.String())
diff --git a/mod.go b/mod.go
index aae3ea9..3678399 100644
--- a/mod.go
+++ b/mod.go
@@ -23,6 +23,10 @@ import (
 	"syscall"
 )
 
+var (
+	ErrNotFound = syscall.ENOENT
+)
+
 // -----------------------------------------------------------------------------
 
 func FindGoMod(dirFrom string) (dir, file string, err error) {
@@ -41,7 +45,7 @@ func FindGoMod(dirFrom string) (dir, file string, err error) {
 			break
 		}
 	}
-	err = syscall.ENOENT
+	err = ErrNotFound
 	return
 }
 
diff --git a/modfetch/fetch.go b/modfetch/fetch.go
index 57f15bc..169a9d4 100644
--- a/modfetch/fetch.go
+++ b/modfetch/fetch.go
@@ -27,8 +27,8 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
-	"syscall"
 
+	xmod "github.com/goplus/mod"
 	"github.com/goplus/mod/modcache"
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
@@ -172,7 +172,7 @@ func Get(modPath string, noCache ...bool) (mod module.Version, err error) {
 	}
 	if noCache == nil || !noCache[0] {
 		mod, err = getFromCache(modPath)
-		if err != syscall.ENOENT {
+		if err != xmod.ErrNotFound {
 			return
 		}
 	}
@@ -190,7 +190,7 @@ func Get(modPath string, noCache ...bool) (mod module.Version, err error) {
 	cmd.Run()
 	if stderr.Len() > 0 {
 		mod, err = getResult(stderr.String())
-		if err != syscall.ENOENT {
+		if err != xmod.ErrNotFound {
 			if debugVerbose {
 				log.Println("modfetch.Get ret:", err)
 			}
@@ -212,7 +212,7 @@ func getResult(data string) (mod module.Version, err error) {
 		}
 		return getMod(data[len(downloading):], nil)
 	}
-	err = syscall.ENOENT
+	err = xmod.ErrNotFound
 	return
 }
 
@@ -227,7 +227,7 @@ func getMod(data string, next *string) (mod module.Version, err error) {
 			return
 		}
 	}
-	err = syscall.ENOENT
+	err = xmod.ErrNotFound
 	return
 }
 
@@ -252,7 +252,7 @@ func lookupFromCache(modPath string) (modRoot string, mod module.Version, err er
 	if pos > 0 { // has version
 		fi, e := os.Stat(modRoot)
 		if e != nil || !fi.IsDir() {
-			err = syscall.ENOENT
+			err = xmod.ErrNotFound
 		}
 		return
 	}
@@ -262,7 +262,7 @@ func lookupFromCache(modPath string) (modRoot string, mod module.Version, err er
 		err = errors.Unwrap(err)
 		return
 	}
-	err = syscall.ENOENT
+	err = xmod.ErrNotFound
 	for _, fi := range fis {
 		if fi.IsDir() {
 			if name := fi.Name(); strings.HasPrefix(name, fname) {