Compiler cannot resolve Union typeclass


I have a piece of code that does something provided one halogen slots type given extends the other. But when I try to use it, I get an error:

No type class instance was found for
    Prim.Row.Union slots2
                   ( subMenu :: Slot Query action4 Int
                   | slots2

I’d say that it should resolve t3 ~ ( subMenu :: Slot Query action4 Int ) and we’re done. Am I missing something here?

It comes from the call site of a function

  :: forall action slots slots' m
   . Union slots _ slots'
  => H.ComponentHTML action slots m
  -> H.ComponentHTML action slots' m
extendHTMLSlots = unsafeCoerce
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