diff --git a/txn_writer.go b/txn_writer.go
index 97d38fe823..43420dc52b 100644
--- a/txn_writer.go
+++ b/txn_writer.go
@@ -530,6 +530,7 @@ fail:
 	return failerr
 }
 
+// calculatePrune returns the paths of the packages to be deleted from vendorDir.
 func calculatePrune(vendorDir string, keep []string, logger *log.Logger) ([]string, error) {
 	if logger != nil {
 		logger.Println("Calculating prune. Checking the following packages:")
@@ -547,7 +548,7 @@ func calculatePrune(vendorDir string, keep []string, logger *log.Logger) ([]stri
 			return nil
 		}
 
-		name := strings.TrimPrefix(path, vendorDir+"/")
+		name := strings.TrimPrefix(path, vendorDir+string(filepath.Separator))
 		if logger != nil {
 			logger.Printf("  %s", name)
 		}
diff --git a/txn_writer_test.go b/txn_writer_test.go
index c0f0b9187a..53abb33695 100644
--- a/txn_writer_test.go
+++ b/txn_writer_test.go
@@ -8,9 +8,12 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 	"testing"
 
+	"reflect"
+
 	"github.com/golang/dep/internal/test"
 	"github.com/pkg/errors"
 )
@@ -567,3 +570,35 @@ func TestSafeWriter_VendorDotGitPreservedWithForceVendor(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestCalculatePrune(t *testing.T) {
+	h := test.NewHelper(t)
+	defer h.Cleanup()
+
+	vendorDir := "vendor"
+	h.TempDir(vendorDir)
+	h.TempDir(filepath.Join(vendorDir, "github.com/keep/pkg/sub"))
+	h.TempDir(filepath.Join(vendorDir, "github.com/prune/pkg/sub"))
+
+	toKeep := []string{
+		filepath.FromSlash("github.com/keep/pkg"),
+		filepath.FromSlash("github.com/keep/pkg/sub"),
+	}
+
+	got, err := calculatePrune(h.Path(vendorDir), toKeep, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sort.Sort(byLen(got))
+
+	want := []string{
+		h.Path(filepath.Join(vendorDir, "github.com/prune/pkg/sub")),
+		h.Path(filepath.Join(vendorDir, "github.com/prune/pkg")),
+		h.Path(filepath.Join(vendorDir, "github.com/prune")),
+	}
+
+	if !reflect.DeepEqual(want, got) {
+		t.Fatalf("calculated prune paths are not as expected.\n(WNT) %s\n(GOT) %s", want, got)
+	}
+}