diff --git a/hclsyntax/expression.go b/hclsyntax/expression.go
index 41c8b77..63f2e88 100644
--- a/hclsyntax/expression.go
+++ b/hclsyntax/expression.go
@@ -1139,13 +1139,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 				// Extract and merge marks from the include expression into the
 				// main set of marks
 				includeUnmarked, includeMarks := include.Unmark()
-				if marks == nil {
-					marks = includeMarks
-				} else {
-					for k := range includeMarks {
-						marks[k] = struct{}{}
-					}
-				}
+				marks = append(marks, includeMarks)
 				if includeUnmarked.False() {
 					// Skip this element
 					continue
@@ -1296,13 +1290,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 				// Extract and merge marks from the include expression into the
 				// main set of marks
 				includeUnmarked, includeMarks := include.Unmark()
-				if marks == nil {
-					marks = includeMarks
-				} else {
-					for k := range includeMarks {
-						marks[k] = struct{}{}
-					}
-				}
+				marks = append(marks, includeMarks)
 				if includeUnmarked.False() {
 					// Skip this element
 					continue