diff --git a/saqc/core/core.py b/saqc/core/core.py
index 605ed7c29140c7999f06864ed221d081a012da9e..0a82f976402ce7a36a66e3d19ca492a8c1a12467 100644
--- a/saqc/core/core.py
+++ b/saqc/core/core.py
@@ -102,8 +102,8 @@ def runner(config_file, flagger, data, flags=None, nodata=np.nan, error_policy="
             start_date = configrow[Fields.START]
             end_date = configrow[Fields.END]
 
-            flag_test = testcol[idx]
-            if pd.isnull(flag_test):
+            func = testcol[idx]
+            if pd.isnull(func):
                 continue
 
             if varname not in data and varname not in flagger.getFlags():
@@ -118,14 +118,14 @@ def runner(config_file, flagger, data, flags=None, nodata=np.nan, error_policy="
             try:
                 # actually run the tests
                 dchunk_result, flagger_chunk_result = evalExpression(
-                    flag_test,
+                    func,
                     data=dchunk,
                     field=varname,
                     flagger=flagger_chunk,
                     nodata=nodata,
                 )
             except Exception as e:
-                if _handleErrors(e, configrow, flag_test, error_policy):
+                if _handleErrors(e, configrow, func, error_policy):
                     raise e
                 continue
 
@@ -137,7 +137,7 @@ def runner(config_file, flagger, data, flags=None, nodata=np.nan, error_policy="
                 flagger_chunk_result,
                 varname,
                 configrow[Fields.PLOT],
-                flag_test,
+                func,
             )
 
     plotAllHook(data, flagger)
diff --git a/saqc/funcs/harm_functions.py b/saqc/funcs/harm_functions.py
index d5bb8f577946ca246b114420b4ff71a62f2059c6..ea3f09691f4b640f65cc9b29c8302981b111cc3e 100644
--- a/saqc/funcs/harm_functions.py
+++ b/saqc/funcs/harm_functions.py
@@ -736,8 +736,9 @@ def _toMerged(
     data.drop(fieldname, axis="columns", errors="ignore", inplace=True)
     flags.drop(fieldname, axis="columns", errors="ignore", inplace=True)
 
-    # first case: there is no data, the data-to-insert would have to be merged with, and also are we not deharmonizing:
-    if (flags.empty) & (target_index is None):
+    # first case: there is no data, the data-to-insert would have
+    # to be merged with, and also are we not deharmonizing:
+    if (data.empty) & (target_index is None):
         return data_to_insert.to_frame(name=fieldname), flagger_to_insert
 
     # if thats not the case: generate the drop mask for the remaining data:
diff --git a/saqc/lib/plotting.py b/saqc/lib/plotting.py
index 68f59d3a0615e07e63867661add89c0c55bb681a..cded3715579d6975230327d931de92d49fcd7193 100644
--- a/saqc/lib/plotting.py
+++ b/saqc/lib/plotting.py
@@ -20,16 +20,16 @@ def plotAllHook(data, flagger):
         _plot(data, flagger, True, __plotvars)
 
 
-def plotHook(data, old, new, varname, do_plot, flag_test, plot_nans=False):
-    # old/new: flagger
+def plotHook(data, flagger_old, flagger_new, varname, do_plot, flag_test, plot_nans=False):
+    # flagger_old/flagger_new: flagger
     if do_plot:
         __plotvars.append(varname)
         # cannot use getFlags here, because if a flag was set (e.g. with force) the
         # flag may be the same, but any additional row (e.g. comment-field) would differ
-        mask = (old._flags[varname] != new._flags[varname])
+        mask = (flagger_old._flags[varname] != flagger_new._flags[varname])
         if isinstance(mask, pd.DataFrame):
             mask = mask.any(axis=1)
-        _plot(data, new, mask, varname, title=flag_test, plot_nans=plot_nans)
+        _plot(data, flagger_new, mask, varname, title=flag_test, plot_nans=plot_nans)
 
 
 def _plot(