Castro, who had only reluctantly agreed to face unscripted questions alongside US President Barack Obama as part of the latter's historic visit to Havana in March, reacted with an angry denial.
"After this meeting is over, you can give me a list of political prisoners, and if we have those political prisoners, they will all be released before the night ends," he said.
Elizardo Sanchez would have loved to have been there at that moment to hand over his list.
Sanchez heads the Cuban Human Rights Committee (CCDH), a banned-but-tolerated organization that compiles an annual list of what it calls political prisoners on the communist island.
It is the only group that attempts to keep such a list in a country where trials are held behind closed doors, the lawyers are all paid by the government and sentences are rarely made public.
Sanchez himself has spent eight years in prison across three separate sentences imposed for his political activities, he says.
His list currently names 93 prisoners being held for "political reasons or charged and sentenced in politically motivated trials."
He says 51 of them are "prisoners of conscience" and 31 held for crimes against the state. Another 11 are on parole after being convicted during the so-called Black Spring, a wave of crack-downs on dissidents in 2003.
Hard to verify
The government and state media, however, question why the list includes people who are out on parole, as well as prisoners convicted for espionage, terrorism or trying to flee the island by hijacking boats or planes.
"Those prisoners were sentenced for crimes against the state, which is a political crime," Sanchez responded.
The paroled inmates, meanwhile, have a "sword of Damocles" hanging over their heads and can be put back behind bars anytime, he said.
The crimes of the 51 "prisoners of conscience" include taking part in banned protests, public disorder and civil disobedience.
The longest sentences are 15 years, for Raibel Pacheco Santos and Jose Ortega Amador, convicted in 2014 of "planning anti-government actions."
Others on the list include former diplomat Miguel Alvarez, sentenced to 25 years in 2012 in what is believed to be an espionage case, and ex-spy Claro Fernando Alonso Hernandez, sentenced to 30 years in 1996 for revealing state secrets.
New tactic: brief detentions
The CCDH is respected, but international rights groups such as Amnesty International and Human Rights Watch do not republish its list, saying they would first have to be able to send their own representatives to Cuba to verify it.
The last authorized international visit to Cuban prisons was in 1988.
"When Amnesty International does not recognize prisoners of conscience, it is not because there aren't any... it is simply that we don't have the means to verify all the information," said Louise Tillotson of the group's Americas office.
Foreign governments face the same dilemma.
While US and European officials regularly condemn repression in Cuba, they do not generally give figures on political prisoners, especially since Cuba freed 53 people the United States considered political prisoners as part of the former Cold War enemies' watershed rapprochement in 2014.
Sanchez frequently asks international organizations to verify certain inmates as political prisoners, but says they are far too slow.
"Sometimes we propose a name. A year later they tell us they've validated it, but we have to tell them, 'It's too late, the prisoner has already been released,'" he said.
Others in a position to know stop short of endorsing Sanchez's list wholesale.
"Cuba has political prisoners, but perhaps not as many as some claim," said Michael Shifter, head of the Inter-American Dialogue research center.
In any case, the regime has now shifted tactics, he said: rather than give dissidents long prison terms, it uses "short-term arbitrary detentions."
This approach can derail protests, for example, but draws less international criticism.
"The question of political prisoners becomes less relevant when the state tactic is to avoid long-term imprisonment and multiply short-term arrests," said Amnesty's Tillotson.
Last year Sanchez's group counted 8,616 politically motivated arrests, mostly short-term. In 2014 it registered 8,889.