diff --git a/src/activation.jl b/src/activation.jl
index ab269305..398fe0fe 100644
--- a/src/activation.jl
+++ b/src/activation.jl
@@ -1,3 +1,5 @@
 export σ
 
 σ(x) = 1 ./ (1 .+ exp.(-x))
+
+back!(::typeof(σ), Δ, x) = Δ .* σ(x)./(1.-σ(x))
diff --git a/src/model.jl b/src/model.jl
index 3d5277b3..1591fa3f 100644
--- a/src/model.jl
+++ b/src/model.jl
@@ -5,7 +5,7 @@ export Model, back!, update!, param
 abstract Model
 
 back!(m::Model, ∇) = error("Backprop not implemented for $(typeof(m))")
-update!(m::Model, η) = m
+update!(m, η) = m
 
 # Model parameters