diff --git a/func-math.go b/func-math.go
index 566d4b2..383d79d 100644
--- a/func-math.go
+++ b/func-math.go
@@ -28,8 +28,8 @@ func doAdd(ctx ExprContext, name string, it Iterator) (result any, err error) {
 			if v, err = doAdd(ctx, name, subIter); err != nil {
 				break
 			}
-			if subIter.HasOperation(cleanName) {
-				if _, err = subIter.CallOperation(cleanName, nil); err != nil {
+			if extIter, ok := v.(ExtIterator); ok && extIter.HasOperation(cleanName) {
+				if _, err = extIter.CallOperation(cleanName, nil); err != nil {
 					return
 				}
 			}
@@ -103,8 +103,8 @@ func doMul(ctx ExprContext, name string, it Iterator) (result any, err error) {
 			if v, err = doMul(ctx, name, subIter); err != nil {
 				break
 			}
-			if subIter.HasOperation(cleanName) {
-				if _, err = subIter.CallOperation(cleanName, nil); err != nil {
+			if extIter, ok := v.(ExtIterator); ok && extIter.HasOperation(cleanName) {
+				if _, err = extIter.CallOperation(cleanName, nil); err != nil {
 					return
 				}
 			}