diff --git a/src/borg/crypto/low_level.pyx b/src/borg/crypto/low_level.pyx
index 1f23a126a..aa9d1c3ae 100644
--- a/src/borg/crypto/low_level.pyx
+++ b/src/borg/crypto/low_level.pyx
@@ -580,7 +580,7 @@ cdef class _AEAD_BASE:
                 raise CryptoError('EVP_CIPHER_CTX_ctrl SET IVLEN failed')
             if not EVP_DecryptInit_ex(self.ctx, NULL, NULL, self.enc_key, iv):
                 raise CryptoError('EVP_DecryptInit_ex failed')
-            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_TAG, self.mac_len, <const unsigned char *> idata.buf+hlen):
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_TAG, self.mac_len, <unsigned char *> idata.buf+hlen):
                 raise CryptoError('EVP_CIPHER_CTX_ctrl SET TAG failed')
             rc = EVP_DecryptUpdate(self.ctx, NULL, &olen, <const unsigned char*> idata.buf+aoffset, alen)
             if not rc: