diff --git a/bet/calculateP/calculateP.py b/bet/calculateP/calculateP.py
index a4c25109..ddb7296e 100644
--- a/bet/calculateP/calculateP.py
+++ b/bet/calculateP/calculateP.py
@@ -368,6 +368,7 @@ def estimate_volume(samples, lambda_emulate=None):
     #local_index = range(0+comm.rank, samples.shape[0], comm.size)
     local_index = np.array_split(np.arange(samples.shape[0]),
             comm.size)[comm.rank]
+    local_index = np.array(local_index, dtype='int64')
     lam_vol_local = lam_vol[local_index]
 
     return (lam_vol, lam_vol_local, local_index)