diff --git a/meep_adjoint/optimization_problem.py b/meep_adjoint/optimization_problem.py
index 688e4cd..7da5135 100644
--- a/meep_adjoint/optimization_problem.py
+++ b/meep_adjoint/optimization_problem.py
@@ -217,7 +217,7 @@ def __call__(self, beta_vector=None, design=None,
             If need_value or need_gradient is False, then fq or gradf in the return
             tuple will be None.
         """
-        if beta_vector or design:
+        if (beta_vector is not None) or design:
             self.update_design(beta_vector=beta_vector, design=design)
 
         #######################################################################