I need to convert words into binary numbers. With a bit help of yours I got this

blCo::String -> Integer
blCo  x  = num2bin(lett2num x)
blCo (x:xs)
          | x:xs = num2bin(lett2num x):blCo xs

num2lett :: Int ->  Char
num2lett n 
   | (n <= ord 'A') && (n <= ord 'Z')   =  chr(ord 'A'+ n - 1)
   | (n <= ord 'a') && (n <= ord 'Z')   =  chr(ord 'A'+ n - 1)

num2bin :: Integer -> String
num2bin n
  | n >= 0     =  concatMap show (reverse ( n2b n))
  | otherwise  =  error "num2bin: negative number"
  where n2b 0  =  []
        n2b n  =  n `mod` 2 : n2b (n `div` 2) 

he tells me a mistake.I dont undertsand it mistake:

 Couldn't match expected type `Char' against inferred type `String'
    In the first argument of `lett2num', namely `x'
    In the first argument of `num2bin', namely `(lett2num x)'
    In the expression: num2bin (lett2num x)