[Solved] Mysql exec_first query_first behave differently

Helo, I'm very newbie in rust.
I'm doing some exercises with rust & mysql but I have a problem.
If I get a row from a table with conn.query_first(.....).map(....) work fine
but If try with the same select instruction with conn.exec_first(....).map
I get error:
thread 'main' panicked at 'Couldn't convert Row.
Dump show that the row is correctly read from mysql.

Can you help me?

I sI solved by my self. After a few tests seems that the two function have a different behavior on map mysql double field.
query_first map on f32
exec_first map on f64.

I hope this can help someone else