Type check fails after inlining a variable


I’m using Slick for database access in a project and stumbled upon a strange type check error, that only happens when I inline a variable. I can reproduce it with the “Hello Slick” example project:

val mySuppliers = Seq( /* ... some entries ... */ )

// works fine
val upserts = mySuplliers.map(suppliers.insertOrUpdate)
val query = DBIO.seq(
  coffees += ("Colombian", 101, 7.99, 0, 0),

// fails with
// inferred type arguments [Nothing,Any,slick.dbio.Effect] do not conform
// to method sequence's type parameter bounds [R,M[+_] <: TraversableOnce[_],E <: slick.dbio.Effect]
val query = DBIO.seq(
  coffees += ("Colombian", 101, 7.99, 0, 0),

If my understanding is correct, these two should be equivalent? I’d understand the need for the extra variable, if it had a type annotation, but here I don’t seem to be doing anything that helps inferring the type…
Also, the error does not occur, if I remove the coffees line in the DBIO.seq.
How do you know, when a separate variable is needed?