Okay Ive been "Germaning" for quite a while and this confused me... but the like link below makes it "easy"; because it's "ein" it changes to -er
Why didn't I chose a simple language like Sanskrit!
Declension is not as simple as that. You have recognized that the linking verb, sein has been used. That means that the predicate phrase is Nominativ also. Because an indefinite article has been used, weak inflection is necessary.
(And, again, "Gro
Ber" is not a word. Use "ss" as a substitute for "ß" if you can't use one of these techniques to type the correct letter.)