diff --git a/src/parser.ts b/src/parser.ts
index 9d336fb4..1a53aedf 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -406,7 +406,7 @@ function tokens2asc(tokens: Token[], assert = defaultAssert()) {
         x.lhs = tokens[i + 3];
         x.rhs = tokens[i + 1];
       }
-      if (gettok(i, 1) == "/" && gettok(i + 4, 0) == "mod") {
+      if (gettok(i, 1) == "/" && tokens[i + 4] && gettok(i + 4, 0) == "mod") {
         x.op = "op%";
         i += 5;
       } else {