MATLAB: Matlab exponent bug


Hello MatLabers
I have encountered a problem when calculating a non integer exponent/power of a variable.
>> -3.^(1.3)
>> ans = -4.1712
thats exactly what I aim to calculate. However, if I do the exact same thing with a variable – the result gets complex:
>> a = -3
>> a.^(1.3)
>> ans = -2.4518 - 3.3745i
is this an known issue or am I doing somehting wrong? (tested on R2010a, R2011b)