undef Error while using (>>=)

[Kleidukos]

Hi! I am currently trying out Hamler and produced the following code:

module Main where                                                      
                                                                       
import Prelude                                                         
import Data.Maybe                                                      
import Data.String as String                                           
                                                                       
main :: IO ()                                                          
main = print "Let there be Hamler, running on Erlang VM!"              
                                                                       
checkPasswordLength :: String -> Maybe String                          
checkPasswordLength password =                                         
  case (String.length password > 20) of                                
       true  -> Nothing                                                
       false -> Just password                                          
                                                                       
cleanWhitespace :: String -> Maybe String                              
cleanWhitespace string =                                               
  case (isSpace x) of                                                  
      true  -> cleanWhitespace xs                                      
      false -> Just $ x <> xs                                          
  where                                                                
    (x, xs) = (String.take 1 string, String.drop 1 string)             
                                                                       
isSpace :: String -> Bool                                              
isSpace " "  = true                                                    
isSpace "\t" = true                                                    
isSpace "\n" = true                                                    
isSpace _    = false                                                   

(Purely for educational purposes).

I tried to run the following code in the REPL, but then got this error:

checkPasswordLength "test"  >>= cleanWhitespace 
Error: error, Reason: undef
Stacktrace:[{'Main',checkPasswordLength,["test"],[]},
            {'$PSCI',it,0,[]},
            {replsrv__escript__1591__956394__873330__5,reploop,1,
                [{file,"/usr/local/lib/hamler/bin/replsrv"},{line,31}]},
            {escript,run,2,[{file,"escript.erl"},{line,758}]},
            {escript,start,1,[{file,"escript.erl"},{line,277}]},
            {init,start_em,1,[]},
            {init,do_boot,3,[]}]