diff --git a/src/heap.rs b/src/heap.rs
index 8dbb9716..1c55a9d4 100644
--- a/src/heap.rs
+++ b/src/heap.rs
@@ -22,7 +22,7 @@ pub struct Heap(Vec<u8>);
 
 impl Heap {
     pub fn reserve(&mut self, additional: usize) {
-        self.0.reserve(additional);
+        self.0.reserve_exact(additional);
     }
 }