Core Data crash coerceValueForKeyWithDescription

A couple of users have been getting these odd crashes that I can't reproduce on my end. This happens the first time they try to create anything in my app, then never again. But it happens once for each type of entity. Here's an excerpt of the crash log:

Frame 5 of the last exception backtrace is the user pushing the save button, which takes a generic view model that conforms to the EditorViewModel protocol, like so:

Code Block Swift
struct AddSaveNavigationBarItem<T: EditorViewModel>: View {
    @ObservedObject var viewModel: T
    @Binding var presentationMode: PresentationMode
    var body: some View {
        Button(saveAddButtonTitle) {
            if let assignmentEditorViewModel = viewModel as? AssignmentEditorViewModel {
                guard !assignmentEditorViewModel.shouldWarnDenominator() else {
                    return
                }
                
                guard !assignmentEditorViewModel.shouldWarnDisableGrades() else {
                    return
                }
            }
            viewModel.save()
            presentationMode.dismiss()
        }
        .disabled(viewModel.name.isEmpty)
    }
}

The next frame is the save method on the view model which saves the changes made to the assignment entity in Core Data:

Code Block Swift
    func save() {
        assignment.name = name
        assignment.course = course
        assignment.isTest = isTest
        assignment.isDueOnDate = isDueOnDate
        assignment.isDueAtTime = isDueAtTime
        if !isDueAtTime && isDueOnDate {
            assignment.dueDate = Calendar.current.date(bySettingHour: 23, minute: 59, second: 59, of: dueDate)
        } else {
            assignment.dueDate = dueDate
        }
        assignment.notes = notes
        if willRecieveGrade {
            assignment.gradeNumerator = Double(gradeNumerator) ?? Assignment.defaultNumerator
            assignment.gradeDenominator = Double(gradeDenominator) ?? Assignment.defaultDenominator
            if !isUsingCustomWeight && weightCategory != nil {
                assignment.weight = Assignment.defaultWeight
                assignment.weightCategory = weightCategory!
            } else {
                assignment.weight = Double(weight) ?? Assignment.defaultWeight
                assignment.weightCategory = nil
            }
        } else {
            assignment.gradeNumerator = Assignment.defaultNumerator
            assignment.gradeDenominator = Assignment.defaultDenominator
            assignment.weight = Assignment.defaultWeight
            assignment.weightCategory = nil
        }
        assignment.willRecieveGrade = willRecieveGrade
        do {
            try context.save()
            AssignmentEditorViewModel.logger.info("Saved changes to assignment \(self.assignment.id, privacy: .public)")
        } catch {
            AssignmentEditorViewModel.logger.error("Failed to save context: \(error.localizedDescription, privacy: .public)")
        }
        for subtask in assignment.subtasks {
            subtask.objectWillChange.send()
        }
        assignment.objectWillChange.send()
    }

I'm a pretty new developer, and this app is a pet project of mine so I'm really struggling to figure out what the issue could be, especially since I can't reproduce it on my end. Any suggestions would be much appreciated!