diff --git a/pypop7/optimizers/ds/powell.py b/pypop7/optimizers/ds/powell.py
index 0b821ec72..0c584c7d2 100644
--- a/pypop7/optimizers/ds/powell.py
+++ b/pypop7/optimizers/ds/powell.py
@@ -85,6 +85,9 @@ def _minimize_scalar_bounded(func, bounds,
 def _line_for_search(x0, alpha, lb, ub):
     # this is adopted from https://github.com/scipy/scipy/blob/main/scipy/optimize/_optimize.py
     nonzero, = alpha.nonzero()
+    if len(nonzero) == 0:
+        return (0, 0)
+      
     lb, ub = lb[nonzero], ub[nonzero]
     x0, alpha = x0[nonzero], alpha[nonzero]
     low, high = (lb - x0)/alpha, (ub - x0)/alpha