diff --git a/ch10/sem/sem_env.go b/ch10/sem/sem_env.go
index 2f68c52..07372c9 100644
--- a/ch10/sem/sem_env.go
+++ b/ch10/sem/sem_env.go
@@ -244,7 +244,7 @@ func (ev *SemEnv) Action(element string, input tensor.Tensor) {
 // String returns the string rep of the LED env state
 func (ev *SemEnv) String() string {
 	cpar := ev.CurPara()
-	if cpar == nil {
+	if len(cpar) == 0 {
 		return ""
 	}
 	str := cpar[0]