diff --git a/src/kyupy/wave_sim.py b/src/kyupy/wave_sim.py
index 170445b..d9bf94b 100644
--- a/src/kyupy/wave_sim.py
+++ b/src/kyupy/wave_sim.py
@@ -434,7 +434,7 @@ class WaveSimCuda(WaveSim):
         grid_dim = self._grid_dim(self.sims, self.s_len)
         wave_assign_gpu[grid_dim, self._block_dim](self.c, self.s, self.c_locs, self.ppi_offset)
 
-    def _grid_dim(self, x, y): return cdiv(x, self._block_dim[0]), cdiv(y, self._block_dim[1])
+    def _grid_dim(self, x, y): return cdiv(int(x), self._block_dim[0]), cdiv(int(y), self._block_dim[1])
 
     def c_prop(self, sims=None, seed=1, op_from=0, op_to=None, delta=0):
         sims = min(sims or self.sims, self.sims)