Occasional malloc error when adding notes

[jbmaxwell]

I'm noticing a fairly frequent crash when calling add(note:):

SpliqsTool(50382,0x70000ee42000) malloc: *** error for object 0x600000ce9620: pointer being freed was not allocated
SpliqsTool(50382,0x70000eec5000) malloc: Heap corruption detected, free list is damaged at 0x600000cfaed0
*** Incorrect guard value: 1
SpliqsTool(50382,0x70000ee42000) malloc: *** set a breakpoint in malloc_error_break to debug
SpliqsTool(50382,0x70000eec5000) malloc: *** set a breakpoint in malloc_error_break to debug
SpliqsTool(50382,0x70000f4e9000) malloc: Heap corruption detected, free list is damaged at 0x600000ce7fd0
*** Incorrect guard value: 105553129677296
SpliqsTool(50382,0x70000f4e9000) malloc: *** set a breakpoint in malloc_error_break to debug

I'm calling it from an extension I wrote for merging events onto a given channel:

extension MidiNoteTrack {
    func add(notes: [MidiNote], mergeToChannel ch: Int) {
        for note in notes {
            let n = MidiNote(timeStamp: note.timeStamp, duration: note.duration, note: note.note, velocity: note.velocity, channel: UInt8(ch))
            self.add(note: n)
        }
    }
}

I don't see how my function could be causing it (though I obviously won't rule out the possibility) so I thought I'd report it, just in case. I am calling my function from a DispatchQueue.concurrentPerform, which contains a midiData.writeData(to:) call—could that be the problem?

I'm on macOS 10.15.3, Xcode 11.3.1, Swift 5.1.