diff --git a/pvlib/iotools/acis.py b/pvlib/iotools/acis.py
index 3be16cfa4c..634af07933 100644
--- a/pvlib/iotools/acis.py
+++ b/pvlib/iotools/acis.py
@@ -413,8 +413,8 @@ def get_acis_station_data(station, start, end, trace_val=0.001,
                  'climdiv,valid_daterange,tzo,network')
     }
     df, metadata = _get_acis(start, end, params, map_variables, url, **kwargs)
-    df = df.replace("M", np.nan)
-    df = df.replace("T", trace_val)
+    df = df.mask(df == 'M', np.nan)
+    df = df.mask(df == 'T', trace_val)
     df = df.astype(float)
     return df, metadata
 
diff --git a/tests/iotools/test_sodapro.py b/tests/iotools/test_sodapro.py
index 7105e9ac98..09f5ee38ea 100644
--- a/tests/iotools/test_sodapro.py
+++ b/tests/iotools/test_sodapro.py
@@ -21,7 +21,7 @@
 
 index_verbose = pd.date_range('2020-06-01 12', periods=4, freq='1min',
                               tz='UTC')
-index_monthly = pd.date_range('2020-01-01', periods=4, freq='1M')
+index_monthly = pd.date_range('2020-01-01', periods=4, freq='1ME')
 
 
 dtypes_mcclear_verbose = [