diff --git a/src/dilax/parameter.py b/src/dilax/parameter.py
index 3980a5d..9402a49 100644
--- a/src/dilax/parameter.py
+++ b/src/dilax/parameter.py
@@ -162,7 +162,7 @@ def scale_factor(self, parameter: Parameter, sumw: jax.Array) -> jax.Array:
         gauss_cdf = jnp.broadcast_to(
             self.constraint.cdf(parameter.value), self.lamb.shape
         )
-        return Poisson(self.lamb).inv_cdf(gauss_cdf)  # type: ignore[arg-type]
+        return Poisson(self.lamb).inv_cdf(gauss_cdf) / sumw  # type: ignore[arg-type]
 
 
 class ModifierBase(eqx.Module):