Base64 decoding (atob) of padded strings yields trailing \x00

When decoding base64 encoded strings padded with trailing '=' the '=' seem to get replaced with \x00:

x = btoa('H')     // "SA=="
atob(x)           // "H\x00\x00"
x = btoa('Ha')    // "SGE="
atob(x)           // "Ha\x00"
x = btoa('Hal')   // "SGFs"
atob(x)           // "Hal"

Looks like a glitch in the base64 decoder?