diff --git a/iter-iter.go b/iter-iter.go
index 8b343ee..6f6bd86 100644
--- a/iter-iter.go
+++ b/iter-iter.go
@@ -12,6 +12,8 @@ import (
 	"git.portale-stac.it/go-pkg/expr/scan"
 )
 
+const iterIterType = "IterIter"
+
 type IterIter struct {
 	it       kern.Iterator
 	count    int64
@@ -22,7 +24,13 @@ type IterIter struct {
 }
 
 func NewIterIter(it kern.Iterator, ctx kern.ExprContext, exprs []*scan.Term) (iter kern.Iterator, err error) {
-	iter = &IterIter{it: it, count: 0, index: -1, ctx: ctx, exprList: exprs, current: nil}
+	if ctx == nil {
+		err = fmt.Errorf("context is required for %s", iterIterType)
+	} else if it == nil {
+		err = fmt.Errorf("source iterator is required for %s", iterIterType)
+	} else {
+		iter = &IterIter{it: it, count: 0, index: -1, ctx: ctx, exprList: exprs, current: nil}
+	}
 	return
 }
 
@@ -31,7 +39,7 @@ func (it *IterIter) String() string {
 }
 
 func (it *IterIter) TypeName() string {
-	return "IterIter"
+	return iterIterType
 }
 
 func (it *IterIter) HasOperation(name string) bool {
diff --git a/t_iter-list_test.go b/t_iter-list_test.go
index 6c35417..24ea2dc 100644
--- a/t_iter-list_test.go
+++ b/t_iter-list_test.go
@@ -92,9 +92,10 @@ func TestNewIterList5(t *testing.T) {
 }
 
 func TestNewIterList6(t *testing.T) {
+	ctx := NewSimpleStore()
 	list := kern.NewListA("a", "b", "c", "d")
-	it1, _ := NewIterator(nil, list, nil)
-	it, _ := NewIterator(nil, it1, nil)
+	it1, _ := NewIterator(ctx, list, nil)
+	it, _ := NewIterator(ctx, it1, nil)
 	if item, err := it.Next(); err != nil {
 		t.Errorf("error: %v", err)
 	} else if item != "a" {