Division by zero

Hi NG
I'm actually not really certain, but found an error in Einstein's 'On the electrodynamics of moving bodies' which is quite serious.
See page six, roughly in the middle:
There we find an equation, which says this:
∂τ/∂y= 0
Now, 'tau' is a time belonging to the moving system k.
This system k moves along the x-axis of system K with velocity v, while x- and xsi-axis coincide and etha- and y axis remain parallel.
In other words v_y is permanently zero, or: ∂y=0.
So we have a 'divide by zero' case.
∂τ/∂y is a time value divided by a space value, hence has the form of 1/v.
Because it contains ∂y, the velocity along the y-axis was meant.
But for a straight lateral movement along the x-axis (only) there should be no movement along the y axis, hence ∂y remains zero, because the y-coordinate remains permanently zero, which is, of course, a constant value.
∂τ/∂y could approach a value, however, but if v_y goes to zero, the quotient ∂τ/∂y would go to infinity and NOT to zero (as the equation says).
Iow: this equation '∂τ/∂y= 0' is wrong!
TH