[Merrick]

So, is there a source or is that your self-written opinion?

[neverevergiveup]

There are multiple sources.

To invoke the 5th amendment there must be a reasonable risk of prosecution. For example:
Mason v. United States, 244 U.S. 362 (1917), is a U.S. Supreme Court decision holding that a witness invoking the Fifth Amendment privilege against self-incrimination must show a reasonable, real danger of criminal liability rather than a mere speculative possibility.

From Justia: Prosecution may be precluded by tender of immunity, or by pardon, Brown v. Walker, 161 U.S. 591, 598–99 (1896).