I was surprised by the following code throwing an error:

let mut reg = Handlebars::new();
reg.set_strict_mode(true);

// register template using given name
reg.register_template_string("tpl_1", "hello {{#if (lookup my_vec 1)}}some{{else}}none{{/if}}")?;
let my_vec = vec![Some(1), None];
println!("{}", reg.render("tpl_1", &json!({"my_vec": my_vec}))?);
Ok(())

Error: RenderError { desc: "Value is missing in strict mode", template_name: Some("tpl_1"), line_no: Some(1), column_no: Some(7), cause: None, unimplemented: false }

There are two ways to fix it. Either disable strict mode, or replace template string with the following (note removed parenthesis inside if).

reg.register_template_string("tpl_1", "hello {{#if lookup my_vec 1}}some{{else}}none{{/if}}")?;

I would expect the original code to work, based on handlebars docs.