看看这个matlab程序错在哪了

看看这个matlab程序错在哪了
t=0:0.01:3;
k=2;a=-1.5;b=10;
f=k*exp((a+i*b)*t);
subplot(2,2,1);plot(f,real(f));title('实部');axis([0,3,-2,2]);grid on;
subplot(2,2,2);plot(f,imag(f));title('虚部');axis([0,3,-2,2]);grid on;
subplot(2,2,3);plot(f,abs(f));title('模');axis([0,3,-2,2]);grid on;
subplot(2,2,4);plot(f,angle(f));title('相位角');axis([0,3,-4,4]);grid on;
错误提示Warning:Imaginary parts of complex X and/or Y arguments ignored