Mixing singa with res
Repeatedly in Calvin's criticism of ubiquitarianism he alleges that his opponents violate the cardinal rule of signification: there is a sacramental, not substantial identification of the signa with the res. Thus in the language he employs he regularly objects to the "confusion" and "mixing" of substances, in particular the humanity of Christ with his divinity or the sacramental reality (Christ's body and blood) with its sign (the elements).
Mark Garcia, Life in Christ: Union with Christ and Twofold Grace in Calvin's Theology (Eugene, OR: Wipf and Stock Publishers, 2008), 162