diff --git a/mlair/helpers/statistics.py b/mlair/helpers/statistics.py
index a8ba9795404e9f18a5ddb7449995fdc5a93eb409..3e99357c36d556f093701325964500bf8d46c698 100644
--- a/mlair/helpers/statistics.py
+++ b/mlair/helpers/statistics.py
@@ -440,7 +440,7 @@ class SkillScores:
         """Calculate CASE IV."""
         AI, BI, CI, data, suffix = self.skill_score_pre_calculations(internal_data, observation_name, forecast_name)
         monthly_mean_external = self.create_monthly_mean_from_daily_data(external_data, index=data.index)
-        data = xr.concat([data, monthly_mean_external], dim="type")
+        data = xr.concat([data, monthly_mean_external], dim="type").dropna(dim="index")
         mean, sigma = suffix["mean"], suffix["sigma"]
         mean_external = monthly_mean_external.mean()
         sigma_external = np.sqrt(monthly_mean_external.var())