diff --git a/src/utils.jl b/src/utils.jl
index 2df2797..53ca5e7 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -1,7 +1,7 @@
 
 using AtomsBase 
 using AtomsBase: Atom, FlexibleSystem
-using Unitful: unit, ustrip, Quantity
+using Unitful: unit, uconvert, ustrip, Quantity
 using LinearAlgebra: norm 
 
 export rattle!, 
@@ -109,12 +109,15 @@ not the same as choosing them uniform in cartesian coordinates!).
 If `r` is unitless, then the unit of the system is applied. 
 """
 function rattle!(at::FlexibleSystem, r::Quantity)
+   (length(at.particles) > 0) || return at
+   at_unit = unit(position(at, 1)[1])
+   r = uconvert(at_unit, r)
    for i = 1:length(at.particles)
       p = at.particles[i]
       𝐫ᵢ = p.position 
       T = typeof(ustrip(𝐫ᵢ[1]))
       ui = randn(Vec3{T})
-      p_new = _set_position(p, 𝐫ᵢ + r * ui / norm(ui))
+      p_new = _set_position(p, 𝐫ᵢ + rand(T) * r * ui / norm(ui))
       at.particles[i] = p_new
    end
    return at
diff --git a/test/test_bulk.jl b/test/test_bulk.jl
index 374ff11..0f8e661 100644
--- a/test/test_bulk.jl
+++ b/test/test_bulk.jl
@@ -35,6 +35,7 @@ end
 sys0 = rattle!(bulk(:C, cubic=true) * (2,3,4), 0.1u"Å")
 sys1 = rattle!(bulk(:C, cubic=true) * (2,3,4), 0.1)
 sys2 = rattle!(bulk(:C) * (2,3,4), 0.1)
+rattle!(bulk(:C) * (2,3,4), 0.01u"nm")
 
 X = position(sys1, :)
 Xnew = [ x .+ 0.01u"Å" for x in X ]