ENH: raise ValueError if invalid period freq pass to asfreq when the index of df is a PeriodIndex

pandas/core/resample.py

@@ -2827,6 +2827,16 @@ def asfreq(
         if how is None:
             how = "E"
 
+        if isinstance(freq, str):
+            freq = to_offset(freq, is_period=True)
+            if hasattr(freq, "_period_dtype_code"):
+                freq = freq_to_period_freqstr(freq.n, freq.name)
+            else:
+                raise ValueError(
+                    f"Invalid offset: '{freq.name}' for converting time series "
+                    f"with PeriodIndex."
+                )
+
         if isinstance(freq, BaseOffset):
             if hasattr(freq, "_period_dtype_code"):
                 freq = freq_to_period_freqstr(freq.n, freq.name)

pandas/tests/resample/test_period_index.py

@@ -1032,14 +1032,30 @@ def test_resample_lowercase_frequency_deprecated(
             offsets.BusinessHour(2),
         ],
     )
-    def test_asfreq_invalid_period_freq(self, offset, frame_or_series):
-        # GH#9586
+    def test_asfreq_invalid_period_offset(self, offset, frame_or_series):
+        # GH#55785
         msg = f"Invalid offset: '{offset.base}' for converting time series "
 
         obj = frame_or_series(range(5), index=period_range("2020-01-01", periods=5))
         with pytest.raises(ValueError, match=msg):
             obj.asfreq(freq=offset)
 
+    @pytest.mark.parametrize(
+        "freq",
+        [
+            "2BMS",
+            "2YS-MAR",
+            "2bh",
+        ],
+    )
+    def test_asfreq_invalid_period_freq(self, freq, frame_or_series):
+        # GH#55785
+        msg = f"Invalid offset: '{freq[1:]}' for converting time series "
+
+        obj = frame_or_series(range(5), index=period_range("2020-01-01", periods=5))
+        with pytest.raises(ValueError, match=msg):
+            obj.asfreq(freq=freq)
+
 
 @pytest.mark.parametrize(
     "freq,freq_depr",