select for update error during precompilation

Greetings:

When precompiling the following sentence the precompiler shows the message "FOR UPDATE is not allowed in this context".

EXEC SQL SELECT camp_1 INTO :n1 WHERE llave = :var_llave FOR UPDATE;

It seems to be a problem with the host variables because it precompiles well the following sentence:

EXEC SQL SELECT camp_1 INTO :n1 WHERE llave = 'abcd' FOR UPDATE;

Thanks in advance for your help,

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Jimmy Roman R.
Guayaquil - Ecuador
South America