bugfix in fn merge_tables

src/compiler/utils/string_with_sourcemap.ts

@@ -59,16 +59,18 @@ function merge_tables<T>(
 ): { table: T[]; new_idx: number[] } {
 	const table = original.slice();
 	const new_idx = [];
-	for (let j = 0; j < original.length; j++) {
+	if (extended) {
+		for (let j = 0; j < extended.length; j++) {
 			const current = extended[j];
 			const existing = table.indexOf(current);
-		if (existing < 0) {
+			if (existing == -1) {
 				table.push(current);
 				new_idx[j] = table.length - 1;
 			} else {
 				new_idx[j] = existing;
 			}
 		}
+	}
 	return { table, new_idx };
 }