diff --git a/skimage/io/_plugins/util.py b/skimage/io/_plugins/util.py
index 1bf2c89f..350b43e5 100644
--- a/skimage/io/_plugins/util.py
+++ b/skimage/io/_plugins/util.py
@@ -213,7 +213,7 @@ class ThreadDispatch(object):
             self.chunks.append((img[(height/2):, :, :],
                                stateimg[(height/2):, :, :]))
 
-        for i in range(self.cores):
+        for i in range(len(self.chunks)):
             self.threads.append(ImgThread(func, self.chunks[i][0],
                                           self.chunks[i][1], *args))