diff --git a/heap_manager.go b/heap_manager.go
index bac9d94..09cdf66 100644
--- a/heap_manager.go
+++ b/heap_manager.go
@@ -25,7 +25,7 @@
 
 type iterData struct {
 	iter chan<- *Bar
-	drop chan struct{}
+	drop <-chan struct{}
 }
 
 type pushData struct {
@@ -113,12 +113,12 @@
 	m <- heapRequest{cmd: h_push, data: data}
 }
 
-func (m heapManager) iter(iter chan<- *Bar, drop chan struct{}) {
+func (m heapManager) iter(iter chan<- *Bar, drop <-chan struct{}) {
 	data := iterData{iter, drop}
 	m <- heapRequest{cmd: h_iter, data: data}
 }
 
-func (m heapManager) drain(iter chan<- *Bar, drop chan struct{}) {
+func (m heapManager) drain(iter chan<- *Bar, drop <-chan struct{}) {
 	data := iterData{iter, drop}
 	m <- heapRequest{cmd: h_drain, data: data}
 }