diff --git a/runtime/JavaScript/src/antlr4/IntervalSet.js b/runtime/JavaScript/src/antlr4/IntervalSet.js
index be7c8b3b8..8a711e301 100644
--- a/runtime/JavaScript/src/antlr4/IntervalSet.js
+++ b/runtime/JavaScript/src/antlr4/IntervalSet.js
@@ -7,11 +7,16 @@ const {Token} = require('./Token');
 
 /* stop is not included! */
 class Interval {
+
 	constructor(start, stop) {
 		this.start = start;
 		this.stop = stop;
 	}
 
+	clone() {
+		return new Interval(this.start, this.stop);
+	}
+
 	contains(item) {
 		return item >= this.start && item < this.stop;
 	}
@@ -55,7 +60,7 @@ class IntervalSet {
 	addInterval(toAdd) {
 		if (this.intervals === null) {
 			this.intervals = [];
-			this.intervals.push(toAdd);
+			this.intervals.push(toAdd.clone());
 		} else {
 			// find insert pos
 			for (let pos = 0; pos < this.intervals.length; pos++) {
@@ -78,7 +83,7 @@ class IntervalSet {
 				}
 			}
 			// greater than any existing
-			this.intervals.push(toAdd);
+			this.intervals.push(toAdd.clone());
 		}
 	}