diff --git a/src/document.cpp b/src/document.cpp
index 099423ad5..8673f6bae 100644
--- a/src/document.cpp
+++ b/src/document.cpp
@@ -422,27 +422,30 @@ SpriteImpl* SpriteImpl::copyLayers(SpriteImpl* dst_sprite, const SpriteImpl* src
 //////////////////////////////////////////////////////////////////////
 // Multi-threading ("sprite wrappers" use this)
 
-bool Document::lock(bool write)
+bool Document::lock(LockType lockType)
 {
   ScopedLock lock(*m_mutex);
 
-  // read-only
-  if (!write) {
-    // If no body is writting the sprite...
-    if (!m_write_lock) {
-      // We can read it
-      ++m_read_locks;
-      return true;
-    }
-  }
-  // read and write
-  else {
-    // If no body is reading and writting...
-    if (m_read_locks == 0 && !m_write_lock) {
-      // We can start writting the sprite...
-      m_write_lock = true;
-      return true;
-    }
+  switch (lockType) {
+
+    case ReadLock:
+      // If no body is writting the sprite...
+      if (!m_write_lock) {
+	// We can read it
+	++m_read_locks;
+	return true;
+      }
+      break;
+
+    case WriteLock:
+      // If no body is reading and writting...
+      if (m_read_locks == 0 && !m_write_lock) {
+	// We can start writting the sprite...
+	m_write_lock = true;
+	return true;
+      }
+      break;
+
   }
 
   return false;
diff --git a/src/document.h b/src/document.h
index 986b8a1c5..2b6dd0ef0 100644
--- a/src/document.h
+++ b/src/document.h
@@ -59,6 +59,11 @@ class Document
 {
 public:
 
+  enum LockType {
+    ReadLock,
+    WriteLock
+  };
+
   // Creates a document with one sprite, with one transparent layer,
   // and one frame.
   static Document* createBasicDocument(int imgtype, int width, int height, int ncolors);
@@ -145,9 +150,9 @@ public:
   //////////////////////////////////////////////////////////////////////
   // Multi-threading ("sprite wrappers" use this)
 
-  // Lock the sprite to read or write it, returning true if the sprite
-  // can be accessed in the desired mode.
-  bool lock(bool write);
+  // Locks the sprite to read or write on it, returning true if the
+  // sprite can be accessed in the desired mode.
+  bool lock(LockType lockType);
 
   // If you've locked the sprite to read, using this method you can
   // raise your access level to write it.
diff --git a/src/document_wrappers.h b/src/document_wrappers.h
index df8117ae2..d87ba710b 100644
--- a/src/document_wrappers.h
+++ b/src/document_wrappers.h
@@ -82,14 +82,14 @@ public:
   explicit DocumentReader(Document* document)
     : DocumentWrapper(document)
   {
-    if (m_document && !m_document->lock(false))
+    if (m_document && !m_document->lock(Document::ReadLock))
       throw LockedDocumentException();
   }
 
   explicit DocumentReader(const DocumentReader& copy)
     : DocumentWrapper(copy)
   {
-    if (m_document && !m_document->lock(false))
+    if (m_document && !m_document->lock(Document::ReadLock))
       throw LockedDocumentException();
   }
 
@@ -102,7 +102,7 @@ public:
     DocumentWrapper::operator=(copy);
 
     // relock the document
-    if (m_document && !m_document->lock(false))
+    if (m_document && !m_document->lock(Document::ReadLock))
       throw LockedDocumentException();
 
     return *this;
@@ -134,7 +134,7 @@ public:
     , m_locked(false)
   {
     if (m_document) {
-      if (!m_document->lock(true))
+      if (!m_document->lock(Document::WriteLock))
 	throw LockedDocumentException();
 
       m_locked = true;