D meson decay and invariant mass

[Narlok]

Considering a D⁰->π⁺K^- where the D meson decays from rest.

If one was to want to calculate the invariant mass of the D meson by measuring the momenta of the pion and kaon, following from conservation of momentum:

m²=(E_π+E_K)²-(p_π+p_K)²

However by inputting numerical data
E_π=137MeV
E_K=493MeV
E_D=1865MeV

As such, since p² is some positive quantity it can only reduce the invariant mass, and as the D meson started at rest the invariant mass should be equal to the rest mass:

(137+493)²<<1865²

Clearly there is a flaw in my understanding, I'm simply not sure where, and any help would be highly appreciated.

 

[Orodruin]

However by inputting numerical data
E_π=137MeV
E_K=493MeV

These are the masses of the pi and K, not their energies.