Fixed calculation of the device_signature.
diff --git a/MagickCore/opencl.c b/MagickCore/opencl.c
index ca92c77..a740f19 100644
--- a/MagickCore/opencl.c
+++ b/MagickCore/opencl.c
@@ -2075,7 +2075,7 @@
       continue;
     }
     device_signature=signature;
-    device_signature=StringSignature(device->platform_name);
+    device_signature^=StringSignature(device->platform_name);
     status=CompileOpenCLKernel(device,accelerateKernelsBuffer,options,
       device_signature,exception);
     UnlockSemaphoreInfo(device->lock);