TST: assert that informative error is raised when offset not supported as a period frequency is passed to DataFrame.asfreq

pandas/core/arrays/period.py

@@ -732,11 +732,16 @@ def asfreq(self, freq=None, how: str = "E") -> Self:
         PeriodIndex(['2010-01', '2011-01', '2012-01', '2013-01', '2014-01',
         '2015-01'], dtype='period[M]')
         """
+        freq_str = freq if isinstance(freq, str) else freq.freqstr
+
         how = libperiod.validate_end_alias(how)
         if isinstance(freq, BaseOffset):
             freq = freq_to_period_freqstr(freq.n, freq.name)
         freq = Period._maybe_convert_freq(freq)
 
+        if not hasattr(freq, "_period_dtype_code"):
+            raise TypeError(f'"{freq_str}" is not supported as a period frequency')
+
         base1 = self._dtype._dtype_code
         base2 = freq._period_dtype_code
 

pandas/tests/frame/methods/test_asfreq.py

@@ -8,6 +8,7 @@
 from pandas import (
     DataFrame,
     DatetimeIndex,
+    PeriodIndex,
     Series,
     date_range,
     period_range,
@@ -257,3 +258,36 @@ def test_asfreq_frequency_M_Q_Y_A_deprecated(self, freq, freq_depr):
         with tm.assert_produces_warning(FutureWarning, match=depr_msg):
             result = df.asfreq(freq=freq_depr)
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "freq, error, error_msg",
+        [
+            (
+                "2MS",
+                TypeError,
+                '"2MS" is not supported as a period frequency',
+            ),
+            (
+                offsets.MonthBegin(),
+                ValueError,
+                (
+                    "Invalid offset: '<MonthBegin>' for converting "
+                    "time series with PeriodIndex."
+                ),
+            ),
+            (
+                offsets.DateOffset(months=2),
+                ValueError,
+                (
+                    "Invalid offset: '<DateOffset: months=2>' for converting "
+                    "time series with PeriodIndex."
+                ),
+            ),
+        ],
+    )
+    def test_asfreq_unsupported_freq(self, freq, error, error_msg):
+        index = PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
+        df = DataFrame({"a": Series([0, 1], index=index)})
+
+        with pytest.raises(error, match=error_msg):
+            df.asfreq(freq=freq)