diff --git a/matlab/backward/backward_model_irf.m b/matlab/backward/backward_model_irf.m
index 79251ade8e0e1f332144a644af121b1e7b384dad..b8b68ab215cd7dc6b669adc00d7f93086cacf71c 100644
--- a/matlab/backward/backward_model_irf.m
+++ b/matlab/backward/backward_model_irf.m
@@ -123,7 +123,7 @@ end
 
 % Get the covariance matrix of the shocks.
 if ~deterministicshockflag
-    if ~nnz(M_.Sigma_e)
+    if nnz(M_.Sigma_e)
         Sigma = M_.Sigma_e + 1e-14*eye(M_.exo_nbr);
         sigma = transpose(chol(Sigma));
     else