diff --git a/OpenCLKernelBuilder/src/main/scala/com/thoughtworks/compute/OpenCLKernelBuilder.scala b/OpenCLKernelBuilder/src/main/scala/com/thoughtworks/compute/OpenCLKernelBuilder.scala
index b3d01a71..0196e7fc 100644
--- a/OpenCLKernelBuilder/src/main/scala/com/thoughtworks/compute/OpenCLKernelBuilder.scala
+++ b/OpenCLKernelBuilder/src/main/scala/com/thoughtworks/compute/OpenCLKernelBuilder.scala
@@ -212,7 +212,7 @@ trait OpenCLKernelBuilder extends FloatArrays {
           }
         }
         val indexId = freshName("index")
-        indexId -> fast"size_t $indexId = ${products.mkFastring(" + ")};\n"
+        indexId -> fast"ptrdiff_t $indexId = ${products.mkFastring(" + ")};\n"
       }).unzip
 
 //      fast"