This fallacy takes the form:
P1. All A’s are B
C. Therefore some B’s are A
Explanation: this fallacy assumes that a category or set has at least one member, even though categories or sets might not have any members. For example, the category ‘popular motion pictures made in the 18th century’ is a logically coherent category (it makes sense to talk about), but it is a category that has no members because no motion pictures were made in the 18th century.
Example: Consider the argument “ghosts are immaterial beings that have no physical form, so therefore at least some beings are immaterial”. Many would say that the first part of this statement falsely asserts that ghosts exist, and therefore the argument is unsound. However, strictly speaking this is not the case. The statement “ghosts are immaterial beings that have no physical form” is quite true, insomuch as this describes an essential property of what is meant by the word ‘ghost’. The problem with this argument, therefore, lies not in the premise but in how it infers its conclusion. In inferring that ‘at least some beings are immaterial’, it is implicitly assumed that at least one ghost exists. If this were true, then the argument would be valid. However, the argument has not established that the category of ‘ghosts’ has any members (i.e. that ghosts actually exist), and as such this argument is an instance of the existential fallacy.
Existential fallacy: overview from Fallacy Files
Existential fallacy: overview from Logically Fallacious
Existential instantiation fallacy: overview from philosophy-index.com