Could not match type

Error output

  Could not match type
                          
    { "third" :: Map t0 t1
    | t2                  
    }                     
                          
  with type
         
    Model
         

while checking that type Model
  is at least as general as type { "third" :: Map t0 t1
                                 | t2                  
                                 }                     
while checking that expression firstModel
  has type { "third" :: Map t0 t1
           | t2                  
           }                     
while checking type of property accessor firstModel."third"
in value declaration semigroupModel

where t1 is an unknown type
      t0 is an unknown type
      t2 is an unknown type

Main.purs

newtype Model = Model
              { first    :: Map String String
              , second      :: Map String String
              , third     :: Map String String
              , fourth   :: Map String String
              }

instance semigroupModel :: Semigroup Model where
  append firstModel secondModel =
    let newFirst = union firstModel.first secondModel.first
        newSecond = union firstModel.second secondModel.second
        newThird = union firstModel.third secondModel.third
        newFourth = union firstModel.fourth secondModel.fourth
    in  Model { first: newFirst, second: newSecond, third: newThird, fourth: newFourth}