diff --git a/music21/instrument.py b/music21/instrument.py
index 25abce154..43e2c7e45 100644
--- a/music21/instrument.py
+++ b/music21/instrument.py
@@ -200,7 +200,7 @@ def __deepcopy__(self, memo=None):
             new.instrumentIdRandomize()
         return new
 
-    def bestName(self):
+    def bestName(self) -> str|None:
         '''
         Find a viable name, looking first at instrument, then part, then
         abbreviations.
diff --git a/music21/musicxml/m21ToXml.py b/music21/musicxml/m21ToXml.py
index a06cb2c6a..aaa6efb7f 100644
--- a/music21/musicxml/m21ToXml.py
+++ b/music21/musicxml/m21ToXml.py
@@ -1399,7 +1399,7 @@ def __init__(self, score: stream.Score|None = None, makeNotation: bool = True):
 
         self.instrumentList: list[instrument.Instrument] = []
         self.instrumentIdList: list[str|None] = []
-        self.midiChannelList: list[int|None] = []
+        self.midiChannelList: list[int|None] = []  # TODO: just list[int]?
 
         self.parts: list[stream.Part] = []
 
@@ -2596,6 +2596,8 @@ class PartExporter(XMLExporterBase):
             in the :class:`~music21.layout.StaffGroup`, if any. (E.g. if this is
             the left hand, store a reference to the right hand.)''',
     }
+    midiChannelList: list[int|None]  # TODO: just list[int]?
+    meterStream: stream.Stream[meter.TimeSignatureBase]
 
     def __init__(self,
                  partObj: stream.Part|stream.Score|None = None,
@@ -2611,14 +2613,15 @@ def __init__(self,
         self.xmlRoot = Element('part')
 
         if parent is None:
-            self.meterStream: stream.Stream[meter.TimeSignatureBase] = stream.Stream()
+            self.meterStream = stream.Stream[meter.TimeSignatureBase]()
             self.refStreamOrTimeRange = [0.0, 0.0]
             self.midiChannelList = []
             self.makeNotation = True
         else:
+            # else should not be executed.
             self.meterStream = (parent.meterStream
                                 if parent.meterStream is not None
-                                else stream.Stream())  # else should not be executed.
+                                else stream.Stream[meter.TimeSignatureBase]())
             self.refStreamOrTimeRange = parent.refStreamOrTimeRange
             self.midiChannelList = parent.midiChannelList  # shared list
             self.makeNotation = parent.makeNotation
@@ -2626,7 +2629,7 @@ def __init__(self,
         self.previousPartStaffInGroup: stream.PartStaff|None = None
 
         self.instrumentStream: stream.Stream[instrument.Instrument]|None = None
-        self.firstInstrumentObject = None
+        self.firstInstrumentObject: instrument.Instrument|None = None
 
         # keep track of this so that we only put out new attributes when something
         # has changed
@@ -2737,16 +2740,19 @@ def instrumentSetup(self):
         else:
             # get a default instrument if not assigned
             self.instrumentStream = self.stream.getInstruments(returnDefault=True, recurse=True)
-        self.firstInstrumentObject = self.instrumentStream[0]  # store first, as handled differently
+        firstInstrumentObject = self.instrumentStream[0]  # store first, as handled differently
+        if t.TYPE_CHECKING:
+            assert isinstance(firstInstrumentObject, instrument.Instrument)
+        self.firstInstrumentObject = firstInstrumentObject
 
         if self.parent is not None:
             instIdList = [x.partId for x in self.parent.instrumentList]
         else:
             instIdList = [self.stream.id]
 
-        firstInstId = self.firstInstrumentObject.partId
+        firstInstId = firstInstrumentObject.partId
         if firstInstId in instIdList or firstInstId is None:  # must have unique ids
-            self.firstInstrumentObject.partIdRandomize()  # set new random id
+            firstInstrumentObject.partIdRandomize()  # set new random id
 
         should_short_circuit = self.mergeInstrumentStreamPartStaffAware()
         if should_short_circuit:
@@ -2769,6 +2775,7 @@ def instrumentSetup(self):
 
             # this is shared among all PartExporters, so long as they are created by a
             # ScoreExporter
+            # TODO: skip if .midiChannel is None?
             self.midiChannelList.append(thisInstrument.midiChannel)
             # environLocal.printDebug(['midiChannel list', self.midiChannelList])
 
@@ -2781,7 +2788,7 @@ def instrumentSetup(self):
             # add to the lists for checking on next part
             if self.parent is not None:
                 self.parent.instrumentIdList.append(thisInstrument.instrumentId)
-                if thisInstrument is self.firstInstrumentObject:
+                if thisInstrument is firstInstrumentObject:
                     self.parent.instrumentList.append(thisInstrument)
 
     def mergeInstrumentStreamPartStaffAware(self) -> bool: