I am trying to solve a piecewise differential equation of the following form:

FOI := diff(r(x), x) = piecewise(r(x) < 0, 1, r(x) > 0, 0)
dsolve({FOI, r(0) = -1})

but MAPLE returns an error stating

Error, (in dsolve) when calling '`property/ConvertProperty`'. Received: 'real is an invalid property'

I do not know what to make of this. I have tried playing around with assumptions...