# Coupled ODEs with boundary conditions - DSolve Error

I'm relatively new to mathematica and I can't find the error here. I suspect it has something to do with the fact that vx, vy, vz aren't blue on my mathematica - as in, they already have some assigned values. But I don't know how that would have happened. I get the error "Supplied equations are not differential equations of the given functions."

Code:

DSolve[{vx'[t] == (fx*c^2 - vx[t]*(fx*vx[t] + fy*vy[t] + fz*vz[t]))/(m*(c^2*Sqrt[
1 - (vx[t]^2 + vy[t]^2 + vz[t]^2)/c^2] + vx[t]^2 + vy[t]^2 + vz[t]^2)),
vy'[t] == (fy*c^2 - vy[t]*(fx*vx[t] + fy*vy[t] + fz*vz[t]))/(m*(c^2*Sqrt[
1 - (vx[t]^2 + vy[t]^2 + vz[t]^2)/c^2] + vx[t]^2 + vy[t]^2 + vz[t]^2)),
vz'[t] == (fz*c^2 - vz[t]*(fx*vx[t] + fy*vy[t] + fz*vz[t]))/(m*(c^2*Sqrt[
1 - (vx[t]^2 + vy[t]^2 + vz[t]^2)/c^2] + vx[t]^2 + vy[t]^2 + vz[t]^2)), vx[0] == 1, vy[0] == 1, vz[0] == 1}, {vx, vy, vz},t]

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You can either clear all symbols with eg ClearAll[vx,vy] etc or quit the kernel with Quit. In any case if you do that DSolve doesn't return a solution. –  acl Jul 6 '14 at 21:49
To clarify acl's comment, after clearing these symbols (or restarting the kernel), DSolve will not give any errors, but it will still not be able to solve the system. You can use NDSolve to get a numerical solution when getting an analytic solution is not possible. –  Szabolcs Jul 6 '14 at 22:10
So is the error just because the equation doesn't have an analytic solution, or is there something wrong with my syntax? –  user31415926535897932384626433 Jul 7 '14 at 13:47
Weird. I just opened the .nb again, ran it again without any changes, and got no error this time. What could have caused that?? –  user31415926535897932384626433 Jul 7 '14 at 13:57