diff --git a/saqc/flagger/history.py b/saqc/flagger/history.py
index 65759b25c87155764f509d215835bcfa14c0a28f..551dba6c4645c317496b5c07cbd28d61a0224d58 100644
--- a/saqc/flagger/history.py
+++ b/saqc/flagger/history.py
@@ -160,7 +160,7 @@ class History:
             self.mask[pos] = pd.Series(True, index=s.index, dtype=bool)
 
         if force:
-            touched = np.isfinite(s)
+            touched = s.notna()
             self.mask.iloc[touched, :pos] = False
 
         self.hist[pos] = s
@@ -287,7 +287,7 @@ class History:
         ----------
         index : pd.Index
             the index to reindex to.
-        fill_value_last : float, default 0
+        fill_value_last : float, default UNFLAGGED
             value to fill nan's (UNTOUCHED) in the last column. Defaults to 0 (UNFLAGGED).
 
         Returns