diff --git a/libtransmission/completion.c b/libtransmission/completion.c
index e11bed310..cd83946b6 100644
--- a/libtransmission/completion.c
+++ b/libtransmission/completion.c
@@ -10,6 +10,8 @@
  * $Id$
  */
 
+#include <assert.h>
+
 #include "transmission.h"
 #include "completion.h"
 #include "torrent.h"
@@ -213,12 +215,19 @@ tr_cpMissingBytesInPiece( const tr_completion * cp, tr_piece_index_t piece )
     else {
         size_t haveBytes = 0;
         tr_block_index_t f, l;
+        const size_t pieceByteSize = tr_torPieceCountBytes( cp->tor, piece );
         tr_torGetPieceBlockRange( cp->tor, piece, &f, &l );
-        haveBytes = tr_bitfieldCountRange( &cp->blockBitfield, f, l+1 );
-        haveBytes *= cp->tor->blockSize;
-        if( tr_bitfieldHas( &cp->blockBitfield, l ) )
+        if( f != l ) {
+            /* nb: we don't pass the usual l+1 here to tr_bitfieldCountRange().
+               It's faster to handle the last block separately because its size
+               needs to be checked separately. */
+            haveBytes = tr_bitfieldCountRange( &cp->blockBitfield, f, l );
+            haveBytes *= cp->tor->blockSize;
+        }
+        if( tr_bitfieldHas( &cp->blockBitfield, l ) ) /* handle the last block */
             haveBytes += tr_torBlockCountBytes( cp->tor, l );
-        return tr_torPieceCountBytes( cp->tor, piece ) - haveBytes;
+        assert(  haveBytes <= pieceByteSize );
+        return pieceByteSize - haveBytes;
     }
 }
 
diff --git a/libtransmission/peer-mgr.c b/libtransmission/peer-mgr.c
index 723c5ae4c..9973b9fb6 100644
--- a/libtransmission/peer-mgr.c
+++ b/libtransmission/peer-mgr.c
@@ -2557,7 +2557,7 @@ peerIsSeed( const tr_peer * peer )
     return false;
 }
 
-/* count how many pieces we want that connected peers have */
+/* count how many bytes we want that connected peers have */
 uint64_t
 tr_peerMgrGetDesiredAvailable( const tr_torrent * tor )
 {
@@ -2594,6 +2594,7 @@ tr_peerMgrGetDesiredAvailable( const tr_torrent * tor )
         if( !tor->info.pieces[i].dnd && ( t->pieceReplication[i] > 0 ) )
             desiredAvailable += tr_cpMissingBytesInPiece( &t->tor->completion, i );
 
+    assert( desiredAvailable <= tor->info.totalSize );
     return desiredAvailable;
 }
 
diff --git a/libtransmission/torrent.c b/libtransmission/torrent.c
index 98b8e78a2..e6c566c34 100644
--- a/libtransmission/torrent.c
+++ b/libtransmission/torrent.c
@@ -1261,6 +1261,10 @@ tr_torrentStat( tr_torrent * tor )
 
     tr_torrentUnlock( tor );
 
+    /* test some of the constraints */
+    assert( s->sizeWhenDone <= tor->info.totalSize );
+    assert( s->leftUntilDone <= s->sizeWhenDone );
+    assert( s->desiredAvailable <= s->leftUntilDone );
     return s;
 }