diff --git a/client/llb/fileop.go b/client/llb/fileop.go
index 831c4f381edd..b57b8463d89f 100644
--- a/client/llb/fileop.go
+++ b/client/llb/fileop.go
@@ -5,6 +5,7 @@ import (
 	_ "crypto/sha256" // for opencontainers/go-digest
 	"os"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -801,6 +802,8 @@ func (f *FileOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 
 func normalizePath(parent, p string, keepSlash bool) string {
 	origPath := p
+	// first, normalize everything to unix path
+	p = filepath.ToSlash(p)
 	p = path.Clean(p)
 	if !path.IsAbs(p) {
 		p = path.Join("/", parent, p)
diff --git a/solver/llbsolver/file/backend.go b/solver/llbsolver/file/backend.go
index 293caa1082bd..58b489cb8a28 100644
--- a/solver/llbsolver/file/backend.go
+++ b/solver/llbsolver/file/backend.go
@@ -139,7 +139,7 @@ func docopy(ctx context.Context, src, dest string, action pb.FileActionCopy, u *
 	}
 	destPath, err := cleanPath(action.Dest)
 	if err != nil {
-		return errors.Wrap(err, "cleaning path")
+		return errors.Wrap(err, "cleaning destination path")
 	}
 	if !action.CreateDestPath {
 		p, err := fs.RootPath(dest, filepath.Join("/", action.Dest))