diff --git a/src/main/scala/millfork/compiler/mos/BuiltIns.scala b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
index c613c99f..2f7569a3 100644
--- a/src/main/scala/millfork/compiler/mos/BuiltIns.scala
+++ b/src/main/scala/millfork/compiler/mos/BuiltIns.scala
@@ -260,6 +260,7 @@ object BuiltIns {
         val labelSkip = ctx.nextLabel("ss")
         val labelRepeat = ctx.nextLabel("sr")
         val loop = List(
+          AssemblyLine.immediate(CPX, 0),
           AssemblyLine.relative(BEQ, labelSkip),
           AssemblyLine.label(labelRepeat),
           AssemblyLine.implied(opcode),