diff --git a/include/libtorrent/packet_buffer.hpp b/include/libtorrent/packet_buffer.hpp
index 35f194d5a..bbfc9200a 100644
--- a/include/libtorrent/packet_buffer.hpp
+++ b/include/libtorrent/packet_buffer.hpp
@@ -101,6 +101,9 @@ namespace libtorrent
 	private:
 		void** m_storage;
 		std::size_t m_capacity;
+
+		// this is the total number of elements that are occupied
+		// in the array
 		std::size_t m_size;
 
 		// This defines the first index that is part of the m_storage.
diff --git a/src/packet_buffer.cpp b/src/packet_buffer.cpp
index 299869a4c..da7f91d91 100644
--- a/src/packet_buffer.cpp
+++ b/src/packet_buffer.cpp
@@ -123,10 +123,10 @@ namespace libtorrent {
 		void* old_value = m_storage[idx & (m_capacity - 1)];
 		m_storage[idx & (m_capacity - 1)] = value;
 
-		if (m_size++ == 0)
-		{
-			m_first = idx;
-		}
+		if (m_size == 0) m_first = idx;
+		// if we're just replacing an old value, the number
+		// of elements in the buffer doesn't actually increase
+		if (old_value == 0) ++m_size;
 
 		TORRENT_ASSERT_VAL(m_first <= 0xffff, m_first);
 		return old_value;