diff --git a/pyproject.toml b/pyproject.toml
index 13b6a0e..8850cb7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,8 +24,9 @@ classifiers = [
 ]
 dependencies = [
     "alive_progress<=3.3.0",
-    "ert<=17.0.0", 
+    "ert<=17.0.0",
     "h5py<=3.14.0",
+    "iterative_ensemble_smoother<=0.6.0",
     "mako<=1.3.10",
     "opm<=2025.10"
 ]