diff --git a/lockable.go b/lockable.go
index b844733..100aeea 100644
--- a/lockable.go
+++ b/lockable.go
@@ -2,6 +2,7 @@ package graphvent
 
 import (
   "encoding/json"
+  "fmt"
 )
 
 // A Listener extension provides a channel that can receive signals on a different thread
@@ -168,11 +169,11 @@ func (ext *LockableExt) HandleLockSignal(ctx *Context, source NodeID, node *Node
   switch state {
   case "unlock":
     if ext.Owner == nil {
-      ctx.Send(node.ID, source, NewLockSignal("already_unlocked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_unlocked")))
     } else if source != *ext.Owner {
-      ctx.Send(node.ID, source, NewLockSignal("not_owner"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_owner")))
     } else if ext.PendingOwner == nil {
-      ctx.Send(node.ID, source, NewLockSignal("already_unlocking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_unlocking")))
     } else {
       if len(ext.Requirements) == 0 {
         ext.Owner = nil
@@ -198,11 +199,11 @@ func (ext *LockableExt) HandleLockSignal(ctx *Context, source NodeID, node *Node
   case "unlocking":
     state, exists := ext.Requirements[source]
     if exists == false {
-      ctx.Send(node.ID, source, NewLockSignal("not_requirement"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_requirement")))
     } else if state.Link != "linked" {
-      ctx.Send(node.ID, source, NewLockSignal("node_not_linked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("node_not_linked")))
     } else if state.Lock != "unlocking" {
-      ctx.Send(node.ID, source, NewLockSignal("not_unlocking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_unlocking")))
     }
 
   case "unlocked":
@@ -212,11 +213,11 @@ func (ext *LockableExt) HandleLockSignal(ctx *Context, source NodeID, node *Node
 
     state, exists := ext.Requirements[source]
     if exists == false {
-      ctx.Send(node.ID, source, NewLockSignal("not_requirement"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_requirement")))
     } else if state.Link != "linked" {
-      ctx.Send(node.ID, source, NewLockSignal("not_linked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_linked")))
     } else if state.Lock != "unlocking" {
-      ctx.Send(node.ID, source, NewLockSignal("not_unlocking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_unlocking")))
     } else {
       state.Lock = "unlocked"
       ext.Requirements[source] = state
@@ -247,11 +248,11 @@ func (ext *LockableExt) HandleLockSignal(ctx *Context, source NodeID, node *Node
 
     state, exists := ext.Requirements[source]
     if exists == false {
-      ctx.Send(node.ID, source, NewLockSignal("not_requirement"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_requirement")))
     } else if state.Link != "linked" {
-      ctx.Send(node.ID, source, NewLockSignal("not_linked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_linked")))
     } else if state.Lock != "locking" {
-      ctx.Send(node.ID, source, NewLockSignal("not_locking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_locking")))
     } else {
       state.Lock = "locked"
       ext.Requirements[source] = state
@@ -277,18 +278,18 @@ func (ext *LockableExt) HandleLockSignal(ctx *Context, source NodeID, node *Node
   case "locking":
     state, exists := ext.Requirements[source]
     if exists == false {
-      ctx.Send(node.ID, source, NewLockSignal("not_requirement"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_requirement")))
     } else if state.Link != "linked" {
-      ctx.Send(node.ID, source, NewLockSignal("node_not_linked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("node_not_linked")))
     } else if state.Lock != "locking" {
-      ctx.Send(node.ID, source, NewLockSignal("not_locking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("not_locking")))
     }
 
   case "lock":
     if ext.Owner != nil {
-      ctx.Send(node.ID, source, NewLockSignal("already_locked"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_locked")))
     } else if ext.PendingOwner != nil {
-      ctx.Send(node.ID, source, NewLockSignal("already_locking"))
+      ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_locking")))
     } else {
       owner := source
       if len(ext.Requirements) == 0 {
@@ -335,9 +336,9 @@ func (ext *LockableExt) HandleLinkStartSignal(ctx *Context, source NodeID, node
       }
     } else if exists == true {
       if state.Link == "linking" {
-        ctx.Send(node.ID, source, NewLinkStartSignal("already_linking_req", target))
+        ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_linking_req")))
       } else if state.Link == "linked" {
-        ctx.Send(node.ID, source, NewLinkStartSignal("already_req", target))
+        ctx.Send(node.ID, source, NewErrorSignal(fmt.Errorf("already_req")))
       }
     } else if dep_exists == true {
       ctx.Send(node.ID, source, NewLinkStartSignal("already_dep", target))