mathematica 解微分方程组

In[39]:= DSolve[{x'[t] == r*x[t], x[0] == x0}, x[t], t]
During evaluation of In[39]:= DSolve::deqn: Equation or list of equations expected instead of True in the first argument {(x^\[Prime])[t]==r x[t],True}. >>
Out[39]= DSolve[{Derivative[1][x][t] == r x[t], True}, x[t], t]

怎么解不出来？