ROLLO was a candidate to the second round of the NIST Post-Quantum Cryptography standardization process. In the last update in April 2020, there was a key encapsulation mechanism (ROLLO-I) and a public-key encryption scheme (ROLLO-II). In this paper, we propose an attack to recover the syndrome during the decapsulation process of ROLLO-I. From this syndrome, we explain how to recover the private key. We target two constant-time implementations: the C reference implementation and a C implementation available on GitHub. By getting power measurements during the execution of the Gaussian elimination function, we are able to extract on a single trace each element of the syndrome. This attack can also be applied to the decryption process of ROLLO-II.