diff --git a/instrumenter.go b/instrumenter.go
index fff6a27..28224e1 100644
--- a/instrumenter.go
+++ b/instrumenter.go
@@ -9,11 +9,11 @@ import (
 	"go/printer"
 	"go/token"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"runtime"
 	"sort"
-	"strconv"
 	"strings"
 )
 
@@ -95,7 +95,8 @@ func (i *instrumenter) instrument(srcDir, singleFile, dstDir string) bool {
 			i.instrumentFile(name, file, dstDir)
 		})
 	}
-	i.writeGobcoFiles(dstDir, pkgs)
+
+	i.writeGobcoFiles(srcDir, dstDir, pkgs)
 	return true
 }
 
@@ -574,7 +575,7 @@ var fixedTemplate string
 //go:embed templates/gobco_no_testmain_test.go
 var noTestMainTemplate string
 
-func (i *instrumenter) writeGobcoFiles(tmpDir string, pkgs []*ast.Package) {
+func (i *instrumenter) writeGobcoFiles(srcDir, tmpDir string, pkgs []*ast.Package) {
 	pkgname := pkgs[0].Name
 	fixPkgname := func(str string) string {
 		str = strings.TrimPrefix(str, "//go:build ignore\n// +build ignore\n\n")
@@ -587,7 +588,7 @@ func (i *instrumenter) writeGobcoFiles(tmpDir string, pkgs []*ast.Package) {
 		writeFile(filepath.Join(tmpDir, "gobco_no_testmain_test.go"), fixPkgname(noTestMainTemplate))
 	}
 
-	i.writeGobcoBlackBox(pkgs, tmpDir)
+	i.writeGobcoBlackBox(pkgs, srcDir, tmpDir)
 }
 
 func (i *instrumenter) writeGobcoGo(filename, pkgname string) {
@@ -612,35 +613,46 @@ func (i *instrumenter) writeGobcoGo(filename, pkgname string) {
 	writeFile(filename, sb.String())
 }
 
+// findPackagePath finds import path of a package that srcDir indicates
+func findPackagePath(srcDir string) (string, error) {
+	_, moduleRel, err := findInModule(srcDir)
+	if err != nil {
+		return "", err
+	}
+
+	moduleName, err := getModuleName()
+	if err != nil {
+		return "", err
+	}
+
+	if moduleRel == "." {
+		return moduleName, nil
+	} else {
+		pkgPath := fmt.Sprintf("%s/%s", moduleName, moduleRel)
+		return pkgPath, nil
+	}
+}
+
+func getModuleName() (string, error) {
+	cmd := exec.Command("go", "list", "-m")
+	output, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(output)), nil
+}
+
 // writeGobcoBlackBox makes the function 'GobcoCover' available
 // to black box tests (those in 'package x_test' instead of 'package x')
 // by delegating to the function of the same name in the main package.
-func (i *instrumenter) writeGobcoBlackBox(pkgs []*ast.Package, dstDir string) {
+func (i *instrumenter) writeGobcoBlackBox(pkgs []*ast.Package, srcDir, dstDir string) {
 	if len(pkgs) < 2 {
 		return
 	}
 
-	// Copy the 'import' directive from one of the existing files.
-	pkgName, pkgPath := "", ""
-	for _, pkg := range pkgs {
-		forEachFile(pkg, func(name string, file *ast.File) {
-			for _, imp := range file.Imports {
-				var impName string
-				p, err := strconv.Unquote(imp.Path.Value)
-				ok(err)
-				if imp.Name != nil {
-					impName = imp.Name.Name
-				} else {
-					impName = filepath.Base(p)
-				}
-
-				if impName == pkgs[0].Name {
-					pkgName = impName
-					pkgPath = p
-				}
-			}
-		})
-	}
+	pkgPath, err := findPackagePath(srcDir)
+	ok(err)
+	pkgName := filepath.Base(pkgPath)
 
 	text := "" +
 		"package " + pkgs[0].Name + "_test\n" +
diff --git a/main.go b/main.go
index d03e634..2f415da 100644
--- a/main.go
+++ b/main.go
@@ -205,27 +205,38 @@ func (g *gobco) gopaths() string {
 	return filepath.Join(home, "go")
 }
 
-func (g *gobco) findInModule(dir string) (moduleRoot, moduleRel string) {
-	absDir, err := filepath.Abs(dir)
+func (g *gobco) findInModule(dir string) (string, string) {
+	moduleRoot, moduleRel, err := findInModule(dir)
 	g.check(err)
+	return moduleRoot, moduleRel
+}
+
+// findInModule finds path of moduleRoot and relative path from the moduleRoot to dir
+func findInModule(dir string) (moduleRoot, moduleRel string, err error) {
+	absDir, err := filepath.Abs(dir)
+	if err != nil {
+		return "", "", err
+	}
 
 	abs := absDir
 	for {
 		if _, err := os.Lstat(filepath.Join(abs, "go.mod")); err == nil {
 			rel, err := filepath.Rel(abs, absDir)
-			g.check(err)
+			if err != nil {
+				return "", "", err
+			}
 
 			root := abs
 			if rel == "." {
 				root = dir
 			}
 
-			return root, rel
+			return root, rel, nil
 		}
 
 		parent := filepath.Dir(abs)
 		if parent == abs {
-			return "", ""
+			return "", "", nil
 		}
 		abs = parent
 	}