Question: Problem with LPSolve command


I'm trying to reproduce in Maple an integer linear programming example from Chvatal's book: 

Maximize z= 4x1 + 5x2 + 5x3 + 2x4

Subject to 33x1 + 49x2 +51x3 + 22x4 <= 120,  x= nonnegative integers. 

The optimal solution is: x1 = 2,   x= 1,  x3 = x4 = 0,     z=  13.


In Maple I type: 

 LPSolve( 4x1 + 5x2 + 5x3 + 2x4, {33x1 + 49x2 +51x3 + 22x4 <= 120}, maximize, assume ={nonnegative, integer}); 

but Maple gives me back the following error: 

Warning, problem appears to be unbounded. [0, [x=0, x2 =0, x3 =0, x4 =0]]


I know the example has the abovementioned definite solution since it comes from the textbook. So, what could I be doing wrongly? 

Many thanks for your help. 


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