diff --git a/pyproject.toml b/pyproject.toml
index 83b2676..b837a5c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "FSRS-Optimizer"
-version = "4.20.7"
+version = "4.20.8"
 readme = "README.md"
 dependencies = [
     "matplotlib>=3.7.0",
diff --git a/src/fsrs_optimizer/fsrs_optimizer.py b/src/fsrs_optimizer/fsrs_optimizer.py
index d81f2d2..a91337e 100644
--- a/src/fsrs_optimizer/fsrs_optimizer.py
+++ b/src/fsrs_optimizer/fsrs_optimizer.py
@@ -1549,6 +1549,7 @@ def loss(stability):
                     .apply(cal_stability)
                     .reset_index(drop=True)
                 )
+                analysis_group.dropna(inplace=True)
                 analysis_group.drop_duplicates(subset=[(group_key, "")], inplace=True)
                 analysis_group.sort_values(by=[group_key], inplace=True)
                 rmse = mean_squared_error(