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What's causing mypy to give an `[assignment]` error in this nested for loop?

I started adding types to my (working) solution to Exercism's "Kindergarten Garden" exercise, to learn how typing with python and Mypy (strict) works. While doing so, I ran into a Mypy error that I can't figure out how to solve. Here's the full code:
```python
class Garden:
    SEEDCHAR_TO_SEED = {
        "C": "Clover",
        "G": "Grass",
        "R": "Radishes",
        "V": "Violets",
    }
    # Student list given when "students" isn't provided when initting instance of Garden
    DEFAULT_STUDENTS = [
        "Alice",
        "Bob",
        "Charlie",
        "David",
        "Eve",
        "Fred",
        "Ginny",
        "Harriet",
        "Ileana",
        "Joseph",
        "Kincaid",
        "Larry",
    ]
    # Init the instance of garden (params filled in when called)
    def __init__(self, diagram: str, students: list[str] = DEFAULT_STUDENTS):
        self.diagram: list[str] = diagram.splitlines()
        # create the full seed garden (rather than using seedchars) in init
        self.seed_diagram: list[list[str]] = []
        for row in self.diagram:
            self.seed_diagram.append([self.SEEDCHAR_TO_SEED[seed] for seed in row])
        # Ensure that the list of students is in the correct (i.e. alphabetical) order
        self.students: list[str] = sorted(students)
        self.results: dict[str, list[str]] = {}
        # Populate self.results with keys as each student, and values being empty lists
        for student in self.students:
            self.results[student] = []
        for student in self.students:
            for row in self.seed_diagram:
                for seed in row[
                    self.students.index(student) * 2 : self.students.index(student) * 2
                    + 2
                ]:
                    self.results[student].append(seed)

    def plants(self, student: str) -> list[str]:
        return self.results[student]
```

running `mypy --strict --pretty` on the file gives this error:
```
kindergarten_garden.py:37: error: Incompatible types in assignment (expression has type "List[str]", variable has type "str")  [assignment]
                for row in self.seed_diagram:
                ^
Found 1 error in 1 file (checked 1 source file)
```

I've been running the code in a debugger to try to find the issue, but I don't see any assignments or expressions that could cause the error. All the types look correct. Am I missing something?