diff --git a/src/Canvas.cc b/src/Canvas.cc
index 8ff622a..d487357 100644
--- a/src/Canvas.cc
+++ b/src/Canvas.cc
@@ -133,7 +133,7 @@ toBuffer(void *c, const uint8_t *data, unsigned len) {
  * EIO toBuffer callback.
  */
 
-void
+int
 Canvas::EIO_ToBuffer(eio_req *req) {
   closure_t *closure = (closure_t *) req->data;
 
@@ -142,6 +142,7 @@ Canvas::EIO_ToBuffer(eio_req *req) {
     , toBuffer
     , closure);
 
+  return 0;
 }
 
 /*
diff --git a/src/Canvas.h b/src/Canvas.h
index 18d0747..a2c03b1 100644
--- a/src/Canvas.h
+++ b/src/Canvas.h
@@ -43,7 +43,7 @@ class Canvas: public node::ObjectWrap {
     static void SetHeight(Local<String> prop, Local<Value> val, const AccessorInfo &info);
     static Handle<Value> StreamPNGSync(const Arguments &args);
     static Local<Value> Error(cairo_status_t status);
-    static void EIO_ToBuffer(eio_req *req);
+    static int EIO_ToBuffer(eio_req *req);
     static int EIO_AfterToBuffer(eio_req *req);
     inline cairo_surface_t *surface(){ return _surface; }
     inline uint8_t *data(){ return cairo_image_surface_get_data(_surface); }