diff --git a/sha3/sha3_s390x.go b/sha3/sha3_s390x.go
index c13ec85..259ff4d 100644
--- a/sha3/sha3_s390x.go
+++ b/sha3/sha3_s390x.go
@@ -112,7 +112,7 @@
 		if len(s.buf) == 0 && len(b) >= cap(s.buf) {
 			// Hash the data directly and push any remaining bytes
 			// into the buffer.
-			remainder := len(s.buf) % s.rate
+			remainder := len(b) % s.rate
 			kimd(s.function, &s.a, b[:len(b)-remainder])
 			if remainder != 0 {
 				s.copyIntoBuf(b[len(b)-remainder:])