diff --git a/caiman/source_extraction/cnmf/map_reduce.py b/caiman/source_extraction/cnmf/map_reduce.py
index efe789e4f..178f3e74e 100644
--- a/caiman/source_extraction/cnmf/map_reduce.py
+++ b/caiman/source_extraction/cnmf/map_reduce.py
@@ -454,7 +454,7 @@ def run_CNMF_patches(file_name, shape, params, gnb=1, dview=None,
         YrA_tot *= nA[:, None]
         nB = np.ravel(np.sqrt(B_tot.power(2).sum(0)))
         B_tot /= nB
-        B_tot = np.array(B_tot, dtype=np.float32)
+        B_tot = B_tot.toarray().astype(np.float32)
 #        B_tot = scipy.sparse.coo_matrix(B_tot)
         F_tot *= nB[:, None]