ECPG and SQL function

Hi,
I wrote a sql funtion which accept 1 varchar in and return an int4.
I try to use this function in a C program with ECPG.
The command : "exec sql select myfunction('myargument') into :res" works fine
but :
"mycommand=select myfonction('mychangingargument')
sql prepare myprep from mycommand
and sql exec execute myprep" does not
and
"mycommand=select myfonction('mychangingargument') into :res
sql prepare myprep from mycommand
and sql exec execute myprep using :res" does not any more.

I got the error "Too few argument" or "Too many argument".

thanks for any kind of help.