Glueing strings together for dynamic variable selection

@CONCATENATE(

I'm trying to glue two values together using concatenate, but the function doesn't seem to work for what i'm doing. It returns error code "1200374:  invalid expression return type"

Here is an example: MonthInt returns the number 1 for Jan, 2 for Feb etc.
:
VAR vBaseHours;
VAR vBaseHours1;
VAR Month;

Months_number(

Month = MonthInt;

vBaseHours = 3;

vBaseHours1 = vBaseHours + "Days in Month";

@CONCATENATE(@SUBSTRING(SUBSTRING("&Month", 0));


)
ENDFIX
:
Both @Name and @Substring are suppose to return strings as values, as I interpret the documentation.

Thank you!

Patrick