diff --git a/Code/Mantid/Framework/PythonInterface/plugins/algorithms/Symmetrise.py b/Code/Mantid/Framework/PythonInterface/plugins/algorithms/Symmetrise.py
index 43a813af9cbe..bef5634a2647 100644
--- a/Code/Mantid/Framework/PythonInterface/plugins/algorithms/Symmetrise.py
+++ b/Code/Mantid/Framework/PythonInterface/plugins/algorithms/Symmetrise.py
@@ -94,7 +94,7 @@ def PyExec(self):
         zeros = np.zeros(new_array_len * num_symm_spectra)
         CreateWorkspace(OutputWorkspace=temp_ws_name,
                         DataX=zeros, DataY=zeros, DataE=zeros,
-                        NSpec=num_symm_spectra,
+                        NSpec=np.int64(num_symm_spectra),
                         UnitX=x_unit)
 
         # Copy logs and properties from sample workspace