[rules-users] Unexpected MVEL + operator behavior (string<->double), in consequence block

Using drools 5.1 mvel 2-2.0.14 In consequence (then) block : (dialect strict or not does not make a difference) Trying to sum up some numbers after a multiplication operation. For example : t.out = t.in2*t.in3 + t.in3*t.in4 + t.in2*t.in4; (the object t has all its attributes of the type String), and all the t.in parameters are actually numbers-as-strings. So the multiplication is going on fine, as expected, but the addition (+) sign, is concatenating instead of adding. So the for inputs "1", "2" and "3", the result looks something like : 2.06.03.0  (i.e concetenated) instead of the expected 11.0. What is going worng  How does the + operator decide to do concat instead of doing addition. Any documentation on this + operator logic ? Note : The above works fine when I use MVEL without Drools. Thanks