diff --git a/artiq/compiler/transforms/interleaver.py b/artiq/compiler/transforms/interleaver.py
index 2e719418c..843f474f6 100644
--- a/artiq/compiler/transforms/interleaver.py
+++ b/artiq/compiler/transforms/interleaver.py
@@ -104,15 +104,17 @@ class Interleaver:
                 target_terminator = target_block.terminator()
                 if isinstance(target_terminator, ir.Parallel):
                     target_terminator.replace_with(ir.Branch(source_block))
-                else:
-                    assert isinstance(target_terminator, (ir.Delay, ir.Branch))
+                elif isinstance(target_terminator, (ir.Delay, ir.Branch)):
                     target_terminator.set_target(source_block)
+                else:
+                    assert False
 
                 source_terminator = source_block.terminator()
-
-                if not isinstance(source_terminator, ir.Delay):
+                if isinstance(source_terminator, (ir.Parallel, ir.Branch)):
                     source_terminator.replace_with(ir.Branch(source_terminator.target()))
-                else:
+                elif isinstance(source_terminator, ir.Return):
+                    break
+                elif isinstance(source_terminator, ir.Delay):
                     old_decomp = source_terminator.decomposition()
                     if is_pure_delay(old_decomp):
                         if target_time_delta > 0:
@@ -144,6 +146,8 @@ class Interleaver:
                             source_terminator.interval = iodelay.Const(target_time_delta)
                         else:
                             source_terminator.replace_with(ir.Branch(source_terminator.target()))
+                else:
+                    assert False
 
                 target_block = source_block
                 target_time  = new_target_time