Attempt to index local tmp error for dynamic node (LuaSnip)

Question

I am trying to create a snippet that changes based on the optional lua pattern input, but I am getting an error about a local tmp for the dynamic node. The desired outcome for typing $cup$ is $\cup$ while for $bcup$, $\bigcup_{<>}^{<>} and I get this error when typing $cup$ or $bcup$. My indexing for the dynamic node seems fine. Any clarification on how to fix this, or why I get this for the dynamic node and not the function node would be appreciated.

Snippet:

return{

s({trig = '([b]?)cup', wordTrig = false, regTrig = true, snippetType = 'autosnippet'},  
     fmta("\\\\<>cup<>",  
          {  
               f(function(\_, snip)  
                    if snip.captures\[1] == 'b' then  
                         return 'big'  
                    end  
               end),  
               d(1, function(\_, snip)  
                    if snip.captures\[1] == 'b' then  
                         return sn(nil, { t("_{"), i(1), t("}^{"), i(2), t("}") })  
                    end  
               end),  
          }  
     )   
),
  
}

Error Message:

Error detected while processing TextChangedI Autocommands for "*":                                            
Error executing lua callback: ...hare/nvim/lazy/LuaSnip/lua/luasnip/nodes/dynamicNode.lua:154: attempt to index local 'tmp' (a nil value) 

Answer 1

Apparently, the answer was that function nodes and dynamic nodes had to return a string and snippetnode, so the revised code that works is

return{

s({trig = '([b]?)cup', wordTrig = false, regTrig = true, snippetType = 'autosnippet'},  
     fmta("\\\\<>cup<>",  
          {  
               f(function(\_, snip)  
                    if snip.captures\[1] == 'b' then  
                         return 'big'  
                    else
                         return ''
                    end  
               end),  
               d(1, function(\_, snip)  
                    if snip.captures\[1] == 'b' then  
                         return sn(nil, { t("_{"), i(1), t("}^{"), i(2), t("}") })  
                    else  
                         return sn(nil, {})
                    end  
               end),  
          }  
     )   
),
  
}