Solving ODE numerically in Mathematica - 'ndnum' error?

I'm trying to solve this ODE [itex]R'(t)=\frac{-a}{R(t)^2}[/itex] numerically in Mathematica (a, b are non-zero constants). Here's what I have:

Code:

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NDSolve[{R'[t]==-a/R[t]^2, R[0]==b,

WhenEvent[R[t]==0, end=t; "StopIntegration"]}, R, {t,0,1}]

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It's returning with

Code:

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NDSolve:::ndnum : Encountered non-numerical value for a derivative at t==0.'

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What does it mean there's a non-numerical value for a derivative at t=0 and how do I fix it?