StreamPlot[{x[0], x'[0]} /.
NDSolve[{x''[t] == 1, x'[0] == v0, x[0] == x0}, {x, x'}, {t, 0, 1}],
{x0, -2, 2}, {v0, -2, 2}]
The above code gives me the errors
NDSolve::ndinnt: Initial condition x0 is not a number or a rectangular array of numbers. >>
ReplaceAll::reps: {NDSolve[{(x^[Prime][Prime])[t]==1,(x^[Prime])[0]==v0,x[0]==x0},{x,x^[Prime]},{t,0,1}]} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>
NDSolve::ndinnt: Initial condition x0 is not a number or a rectangular array of numbers. >>
ReplaceAll::reps: {NDSolve[{(x^[Prime][Prime])[t]==1,(x^[Prime])[0]==v0,x[0]==x0},{x,x^[Prime]},{t,0,1}]} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>
While the code still produces the plot that I want, is it possible to get rid of the error messages?