Plugin panics when trying to read line number from SourceMapper

pub struct MyVisitor {
    // Provided by in from TransformPluginProgramMetadata
    source_map: Arc<dyn SourceMapper>,
}

impl MyVisitor {
    fn my_fn(&mut self, node: &mut Expr) -> Result<(), Box<SpanLinesError>> {
        let line_num = self
            .source_map
            .span_to_lines(node.span())
            .map(|lines| lines.lines[0])?; 

        // also using `self.source_map.span_to_snippet` and that part works okay
    }
}

pub struct MyVisitor {
    // Provided by in from TransformPluginProgramMetadata
    source_map: Arc<dyn SourceMapper>,
}

impl MyVisitor {
    fn my_fn(&mut self, node: &mut Expr) -> Result<(), Box<SpanLinesError>> {
        let line_num = self
            .source_map
            .span_to_lines(node.span())
            .map(|lines| lines.lines[0])?; 

        // also using `self.source_map.span_to_snippet` and that part works okay
    }
}