diff --git a/lang/src/son/hbvm/regalloc.rs b/lang/src/son/hbvm/regalloc.rs
index 20828d7..aa47d6e 100644
--- a/lang/src/son/hbvm/regalloc.rs
+++ b/lang/src/son/hbvm/regalloc.rs
@@ -301,10 +301,11 @@ impl HbvmBackend {
         let bundle_count = self.ralloc.bundles.len() + (reg_offset as usize);
         (
             if tail {
+                debug_assert!(bundle_count >= reg::RET_ADDR as _);
+                bundle_count.saturating_sub(reg::RET_ADDR as _)
+            } else {
                 assert!(bundle_count < reg::STACK_PTR as usize, "TODO: spill memory");
                 self.ralloc.bundles.len()
-            } else {
-                bundle_count.saturating_sub(reg::RET_ADDR as _)
             },
             tail,
         )