diff --git a/client/llb/fileop.go b/client/llb/fileop.go
index 831c4f381edd7..e054f128b1fc1 100644
--- a/client/llb/fileop.go
+++ b/client/llb/fileop.go
@@ -801,6 +801,7 @@ 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 = path.Clean(p)
 	if !path.IsAbs(p) {
 		p = path.Join("/", parent, p)
diff --git a/frontend/dockerfile/dockerfile2llb/convert.go b/frontend/dockerfile/dockerfile2llb/convert.go
index ea5e8601536ba..e11da2eb69ebc 100644
--- a/frontend/dockerfile/dockerfile2llb/convert.go
+++ b/frontend/dockerfile/dockerfile2llb/convert.go
@@ -1139,7 +1139,10 @@ func dispatchCopy(d *dispatchState, cfg copyConfig) error {
 	}
 
 	if cfg.params.DestPath == "." || cfg.params.DestPath == "" || cfg.params.DestPath[len(cfg.params.DestPath)-1] == filepath.Separator {
-		dest += string(filepath.Separator)
+		// can use unix path separator since
+		// at this point, Windows dest path is already normalized
+		// in `pathRelativeToWorkingDir` above.
+		dest += "/"
 	}
 
 	var copyOpt []llb.CopyOption
diff --git a/solver/llbsolver/file/backend.go b/solver/llbsolver/file/backend.go
index 293caa1082bd9..58b489cb8a286 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))