Unclear about a subject-verb agreement rule

[macjas]

Consider the following sentence:

The invention of solid-state transistors (was/were) an important milestone in the field of electronics.

According to rule of subject-verb agreement, we would use "was" if we were referring to "invention" and "were" if we were referring to transistors.

My reasoning is that one should use "was" because "of solid-state transistors" is a prepositional phrase and the real subject is "invention"

Is my reasoning correct? Or on the other hand, would the use of either "was" or "were" be considered incorrect since it cannot be reasonably determined what the subject is without more information that clarifies the intent of the writer?

[macjas]

Source: I came up with this example to illustrate this point.

[tim]

you are totally right. the subject cannot occur inside a prepositional phrase..