diff --git a/artiq/compiler/transforms/interleaver.py b/artiq/compiler/transforms/interleaver.py
index 1fc8b4e1e..e1348ecd6 100644
--- a/artiq/compiler/transforms/interleaver.py
+++ b/artiq/compiler/transforms/interleaver.py
@@ -65,8 +65,9 @@ class Interleaver:
                     terminator = block.terminator()
                     if isinstance(terminator, ir.Delay):
                         # We should be able to fold everything without free variables.
-                        assert iodelay.is_const(terminator.expr)
-                        return terminator.expr.value
+                        folded_expr = terminator.expr.fold()
+                        assert iodelay.is_const(folded_expr)
+                        return folded_expr.value
                     else:
                         return 0