diff --git a/src/safepython/util.py b/src/safepython/util.py
index 339c83a..ed473b3 100644
--- a/src/safepython/util.py
+++ b/src/safepython/util.py
@@ -327,7 +327,7 @@ def aggregate_boot(S, alfa=0.05):
     for j in range(R): # loop over sample sizes
 
         S_m[j, :] = np.nanmean(S[j], axis=0) # bootstrap mean
-        idx = ~np.isnan(S[j][:, 1])
+        idx = ~np.isnan(S[j][:, 0])
         if np.sum(idx) < Nboot:
             warn('Statistics were computed using ' + '%d' % (np.sum(idx))+
                  ' bootstrap resamples instead of '+'%d' % (Nboot))
@@ -520,4 +520,4 @@ def allrange(y, par):
 
     idx = np.full(y.shape, True, dtype=bool)
 
-    return idx
\ No newline at end of file
+    return idx