otr.patch

diff --git a/otr/otr.go b/otr/otr.go
index 0d18a60..ed560ed 100644
--- a/otr/otr.go
+++ b/otr/otr.go
@@ -12,20 +12,21 @@ import (
 	"crypto/cipher"
 	"crypto/dsa"
 	"crypto/hmac"
 	"crypto/rand"
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/subtle"
 	"encoding/base64"
 	"encoding/hex"
 	"errors"
+	"fmt"
 	"hash"
 	"io"
 	"math/big"
 	"strconv"
 )
 
 // SecurityChange describes a change in the security state of a Conversation.
 type SecurityChange int
 
 const (
@@ -1030,21 +1031,26 @@ func (c *Conversation) calcDataKeys(myKeyId, theirKeyId uint32) (slot *keySlot,
 
 	// Find an empty slot to write into.
 	slot = nil
 	for i := range c.keySlots {
 		if !c.keySlots[i].used {
 			slot = &c.keySlots[i]
 			break
 		}
 	}
 	if slot == nil {
-		err = errors.New("otr: internal error: no key slots")
+		errStr := fmt.Sprintf("otr: internal error: no key slots (myKeyId: %d, theirKeyId: %d", myKeyId, theirKeyId)
+		for i := range c.keySlots {
+			slot := &c.keySlots[i]
+			errStr += fmt.Sprintf(", slot[%d] = {myKeyId: %d, theirKeyId: %d}", slot.myKeyId, slot.theirKeyId)
+		}
+		err = errors.New(errStr)
 		return
 	}
 
 	var myPriv, myPub, theirPub *big.Int
 
 	if myKeyId == c.myKeyId {
 		myPriv = c.myCurrentDHPriv
 		myPub = c.myCurrentDHPub
 	} else if myKeyId == c.myKeyId-1 {
 		myPriv = c.myLastDHPriv