diff --git a/saqc/funcs/harm_functions.py b/saqc/funcs/harm_functions.py
index 28e4085ac86534a182b6c15567e31543324c97fd..9f96141d1ef884d06db4dca6743fd84f5be4a03b 100644
--- a/saqc/funcs/harm_functions.py
+++ b/saqc/funcs/harm_functions.py
@@ -618,9 +618,9 @@ def _reshapeFlags(
 
         # some consistency clean up to ensure new flags frame matching new data frames size:
         if ref_index[0] != flags.index[0]:
-            flags = pd.Series(data=flagger.BAD, index=[ref_index[0]], dtype=flagger.dtype).append(flags)
+            flags = pd.Series(data=flagger.BAD, index=[ref_index[0]]).astype(flagger.dtype).append(flags)
         if ref_index[-1] != flags.index[-1]:
-            flags = flags.append(pd.Series(data=flagger.BAD, index=[ref_index[-1]], dtype=flagger.dtype))
+            flags = flags.append(pd.Series(data=flagger.BAD, index=[ref_index[-1]]).astype(flagger.dtype))
 
         flagger_new = flagger.initFlags(flags=flags.to_frame(name=field))
 
diff --git a/test/funcs/test_harm_funcs.py b/test/funcs/test_harm_funcs.py
index a74523ceade0c6eeb4c318f9aacd6b5067b9f8eb..2419370e319243a54417d18225c9aba99586a48d 100644
--- a/test/funcs/test_harm_funcs.py
+++ b/test/funcs/test_harm_funcs.py
@@ -291,6 +291,7 @@ def test_multivariatHarmonization(multi_data, flagger, shift_comment):
 def test_gridInterpolation(data, method):
     freq = "15min"
     data = (data * np.sin(data)).append(data.shift(1, "2h")).shift(1, "3s")
+    data = data.squeeze()
     # we are just testing if the interpolation gets passed to the series without causing an error:
     _interpolateGrid(
         data, freq, method, order=1, agg_method=sum, downcast_interpolation=True
@@ -341,9 +342,8 @@ def test_wrapper(data, flagger):
 
 if __name__ == "__main__":
     dat = data()
-    dat = dat.drop(dat.index[1:3])
-    dat2 = dat.copy()
-    flagger = TESTFLAGGER[2]
+    flagger = TESTFLAGGER[1]
+    test_gridInterpolation(data(), 'polynomial')
     flagger2 = TESTFLAGGER[2]
     flagger = flagger.initFlags(dat)
     flagger2 = flagger.initFlags(dat2)