diff --git a/client/llb/exec.go b/client/llb/exec.go
index 2b1d9bd3f..45133772d 100644
--- a/client/llb/exec.go
+++ b/client/llb/exec.go
@@ -339,7 +339,7 @@ func (e *ExecOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 			inputIndex = pb.Empty
 		}
 
-		outputIndex := pb.OutputIndex(-1)
+		outputIndex := pb.SkipOutput
 		if !m.noOutput && !m.readonly && m.cacheID == "" && !m.tmpfs {
 			outputIndex = pb.OutputIndex(outIndex)
 			outIndex++
diff --git a/client/llb/fileop.go b/client/llb/fileop.go
index ffc6da19e..d632f5a27 100644
--- a/client/llb/fileop.go
+++ b/client/llb/fileop.go
@@ -702,7 +702,7 @@ func (f *FileOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 	pop.Inputs = state.inputs
 
 	for i, st := range state.actions {
-		output := pb.OutputIndex(-1)
+		output := pb.SkipOutput
 		if i+1 == len(state.actions) {
 			output = 0
 		}