BUG: raise if invalid freq is passed (pandas-dev#23546)

pandas/_libs/tslibs/timestamps.pyx

@@ -733,11 +733,11 @@ class Timestamp(_Timestamp):
         if ts.value == NPY_NAT:
             return NaT
 
-        if is_string_object(freq):
-            freq = to_offset(freq)
-        elif not is_offset_object(freq):
+        if freq is None:
             # GH 22311: Try to extract the frequency of a given Timestamp input
             freq = getattr(ts_input, 'freq', None)
+        elif not is_offset_object(freq):
+            freq = to_offset(freq)
 
         return create_timestamp_from_ts(ts.value, ts.dts, ts.tzinfo, freq)
 

pandas/tests/plotting/test_converter.py

@@ -285,11 +285,11 @@ def _assert_less(ts1, ts2):
         _assert_less(ts, ts + Micro(50))
 
     def test_convert_nested(self):
-        inner = [Timestamp('2017-01-01', Timestamp('2017-01-02'))]
+        inner = [Timestamp('2017-01-01'), Timestamp('2017-01-02')]
         data = [inner, inner]
         result = self.dtc.convert(data, None, None)
         expected = [self.dtc.convert(x, None, None) for x in data]
-        assert result == expected
+        assert (np.array(result) == expected).all()
 
 
 class TestPeriodConverter(object):

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -575,6 +575,11 @@ def test_construct_timestamp_preserve_original_frequency(self):
         expected = offsets.Day()
         assert result == expected
 
+    def test_constructor_invalid_frequency(self):
+        # GH 22311
+        with tm.assert_raises_regex(ValueError, "Invalid frequency:"):
+            Timestamp('2012-01-01', freq=[])
+
 
 class TestTimestamp(object):