The lease is void because the people who signed the lease on behalf of Cuba has no authority to sign such a document. A lease of land for military purposes was prohibited by the Cuban Constitution in force at the time, and everyone knew it.
The lease between the US and Cuba for property in Cuba is contained in two documents, both executed in 1903.
The text of those documents can be found at
*http://avalon.law.yale.edu/20th_century/dip_cuba002.asp
*http://avalon.law.yale.edu/20th_century/dip_cuba003.asp
The Cuban Constitution contains the following provision, ironically forced into it by the US in the Platt Amendment:
"I.-That the government of Cuba shall never enter into any treaty or other compact with any foreign power or powers which will impair or tend to impair the independence of Cuba, nor in any manner authorize or permit any foreign power or powers to obtain by colonization or for military or naval purposes or otherwise, lodgement in or control over any portion of said island."
Since the US is a foreign power, this provision seems to make illegal the execution of a lease for a naval base, and any Cuban official, signing such a lease, is in violation of his own constitution, from which his powers spring.
An objection might be made that Article Seven makes an exception to Article One, and allows a naval station to be created, as part of a mutual defense pact:
"VII. That to enable the United States to maintain the independence of Cuba, and to protect the people thereof, as well as for its own defense, the government of Cuba will sell or lease to the United States lands necessary for coaling or naval stations at certain specified points to be agreed upon with the President of the United States."
If this is true, there still remains an insurmountable problem. Then the base is part of a mutual defense pact between Cuba and the United States. However, this benefit, of the essence of the lease, has been withheld from Cuba. The base did not help defend Cuba during the Bay of Pigs invasion, and so did not fulfill its promise, and the claim by John Kennedy that the US will not invade Cuba falls short of the commitment to come to its aid, and serves as an anticipatory breach. The lease is void. Promises must be kept.