Was there a preexisting covenant with Noach?

When Hashem first tells Noach he will save him, He says:

יח וַהֲקִמֹתִי אֶת-בְּרִיתִי, אִתָּךְ; וּבָאתָ, אֶל-הַתֵּבָה--אַתָּה, וּבָנֶיךָ וְאִשְׁתְּךָ וּנְשֵׁי-בָנֶיךָ אִתָּךְ.

18 But I will establish My covenant with thee; and thou shalt come into the ark, thou, and thy sons, and thy wife, and thy sons' wives with thee.

Taking it not as establish (as in the JPS translation) but "keep", "sustain," from whence this covenant? Why have we not heard of this covenant before this? Ibn Ezra assumes that there was a preexisting covenant, and that it is Biblical style to sometimes refer to something not yet mentioned, and we know about it now that it has been mentioned. Thus:

ו, יח

והקימותי את בריתי-

לאות שהשם נשבע לו שלא ימות הוא ובניו במבול, ואם לא נמצא בתחילה מפורש כאשר מצאנו במשנה התורה:נשלחה אנשים לפנינו.

ומלת והקימותי שאקיים את שבועתי.

והקרוב אלי שזאת הברית רמז לקשת.

So He is promising to keep His oath, and that oath was to preserve Noach and his family.

But Ibn Caspi does not think this oath / covenant needs to be preexisting, and thinks that the language of the pasuk works out with a covenant being established just then. I tend to agree.

(As an aside, IIRC, Bereishis Rabbasi takes this bris as evidence of circumcision by Noach.)