diff --git a/openmc/data/multipole.py b/openmc/data/multipole.py
index 97038bd72d..4d4359cc2b 100644
--- a/openmc/data/multipole.py
+++ b/openmc/data/multipole.py
@@ -1143,7 +1143,8 @@ class WindowedMultipole(EqualityMixin):
         # the 1-based vs. 0-based indexing.  Similarly startw needs to be
         # decreased by 1.  endw does not need to be decreased because
         # range(startw, endw) does not include endw.
-        i_window = int(np.floor((sqrtE - sqrt(self.E_min)) / self.spacing))
+        i_window = min(self.n_windows - 1,
+                       int(np.floor((sqrtE - sqrt(self.E_min)) / self.spacing)))
         startw = self.windows[i_window, 0] - 1
         endw = self.windows[i_window, 1]
 
diff --git a/src/wmp.cpp b/src/wmp.cpp
index 44f7eb51ad..44b346bb4a 100644
--- a/src/wmp.cpp
+++ b/src/wmp.cpp
@@ -70,7 +70,8 @@ WindowedMultipole::evaluate(double E, double sqrtkT)
   double invE = 1.0 / E;
 
   // Locate window containing energy
-  int i_window = (sqrtE - std::sqrt(E_min_)) / spacing_;
+  int i_window = std::min(windows_.shape()[0] - 1,
+                          (sqrtE - std::sqrt(E_min_)) / spacing_);
   int startw = windows_(i_window, 0) - 1;
   int endw = windows_(i_window, 1) - 1;