ODBC int2 parameters to pg function

I have discovered a problem with passing parameters to a pg function
using the ODBC driver.

I have delcared a pg function as:

function GENERATE_TIMESHEETS(timestamp,timestamp,smallint,smallint)
....

I have a .NET program that uses the following:
cmd.CommandText = "SELECT GENERATE_TIMESHEETS(?,?,?,?)";
cmd.Parameters.Add("DATE_FROM",OdbcType.DateTime,8);
cmd.Parameters.Add("DATE_TO",OdbcType.DateTime,8);
cmd.Parameters.Add("TIME_FROM",OdbcType.SmallInt,2);
cmd.Parameters.Add("TIME_TO",OdbcType.SmallInt,2);

I then assign the parameter values and execute the statement.

pg returns an error stating
GENERATE_TIMESHEETS(timestamp,timestamp,int4,int4) cannot be found

It appears that the ODBC driver is not typecasting the smallint (int2)
parameters correctly for the function call.

Can anyone shed any light on this?

Thanks
Gary.