Full Idea
Frege ascribes reference not only to singular terms, but equally to expressions of other kinds (the various kinds of incomplete expressions).
Gist of Idea
Frege ascribes reference to incomplete expressions, as well as to singular terms
Source
report of Gottlob Frege (On Sense and Reference [1892]) by Bob Hale - Abstract Objects Ch.3 Intro
A Reaction
The incomplete expressions presumably make reference to concepts. Frege may not seem, therefore, to have a notion of reference as what plugs language into reality - except that he is presumably a platonist about concepts.