Calling to_yaml on pyspark dataframemodel leads to 'Column' object has no attribute 'unique'

[pascalwhoop]

Describe the bug
When calling

KGEdgeSchema.to_json() or to_yaml() on

class KGEdgeSchema(DataFrameModel):
    """Schema for a knowledge graph edges as exposed by the Data API."""

    # fmt: off
    subject:                     StringType()            = pa.Field(nullable = False)
    predicate:                   StringType()            = pa.Field(nullable = False)
    object:                      StringType()            = pa.Field(nullable = False)
    knowledge_level:             StringType()            = pa.Field(nullable = True)
    primary_knowledge_source:    StringType()            = pa.Field(nullable = True)
    aggregator_knowledge_source: ArrayType(StringType()) = pa.Field(nullable = True)
    publications:                ArrayType(StringType()) = pa.Field(nullable = True)
    subject_aspect_qualifier:    StringType()            = pa.Field(nullable = True)
    subject_direction_qualifier: StringType()            = pa.Field(nullable = True)
    object_aspect_qualifier:     StringType()            = pa.Field(nullable = True)
    object_direction_qualifier:  StringType()            = pa.Field(nullable = True)
    # We manually set this for every KG we ingest
    upstream_data_source:          ArrayType(StringType()) = pa.Field(nullable = False)
    # fmt: on

    class Config:  # noqa: D106
        coerce = True
        strict = False

AttributeError: 'Column' object has no attribute 'unique'