diff --git a/tests/extmod/deflate_compress.py b/tests/extmod/deflate_compress.py
index 612af663e4..981a986cbf 100644
--- a/tests/extmod/deflate_compress.py
+++ b/tests/extmod/deflate_compress.py
@@ -146,3 +146,9 @@ for wbits in range(5, 10):
     next_len = len(result)
     print(next_len < prev_len and decompress(result, deflate.RAW, wbits) == buf)
     prev_len = next_len
+
+# Verify that compression is optimal: in the bytes below, the final "123" should be
+# compressed by referencing the "123" just before it, and not the one all the way back
+# at the start of the bytes.
+compressed = compress(b"1234567890abcdefghijklmnopqrstuvwxyz123123", deflate.RAW)
+print(len(compressed), compressed)
diff --git a/tests/extmod/deflate_compress.py.exp b/tests/extmod/deflate_compress.py.exp
index 5da70f491b..7b00fa3cb2 100644
--- a/tests/extmod/deflate_compress.py.exp
+++ b/tests/extmod/deflate_compress.py.exp
@@ -39,3 +39,4 @@ True
 True
 True
 True
+41 b'3426153\xb7\xb04HLJNIMK\xcf\xc8\xcc\xca\xce\xc9\xcd\xcb/(,*.)-+\xaf\xa8\xac\x02\xaa\x01"\x00'