ternary operator doesn't work with lambda functions

I am assigning to a std::function<double()> a lambda expression. This snippet works

if(fn_type==exponential)
    k.*variable = [=,&k](){ return initial*exp(-k.kstep*par); };
else
    k.*variable = [=,&k](){ return initial*pow(k.kstep, par); };

whereas if I want to use the ternary operator

k.*variable = (fn_type==exponential ? [=,&k](){ return initial*exp(-k.kstep*par); } : [=,&k](){ return initial*pow(k.kstep, par); });

I get the following error:

error: no match for ternary ‘operator?:’ in <awfully long template error, because this whole thing is in a class defined in a function...>

Is this a gcc bug (I'm using 4.7.2)? Otherwise why is there this limit in the standard?