diff --git a/src/ReverseDiff.jl b/src/ReverseDiff.jl
index 2580aae..caff556 100644
--- a/src/ReverseDiff.jl
+++ b/src/ReverseDiff.jl
@@ -23,7 +23,7 @@ using ChainRulesCore
 
 # Not all operations will be valid over all of these types, but that's okay; such cases
 # will simply error when they hit the original operation in the overloaded definition.
-const ARRAY_TYPES = (:AbstractArray, :AbstractVector, :AbstractMatrix, :Array, :Vector, :Matrix)
+const ARRAY_TYPES = (:AbstractArray, :AbstractVector, :AbstractMatrix, :Array, :Vector, :Matrix, :Diagonal)
 const REAL_TYPES = (:Bool, :Integer, :(Irrational{:ℯ}), :(Irrational{:π}), :Rational, :BigFloat, :BigInt, :AbstractFloat, :Real, :Dual)
 
 const SKIPPED_UNARY_SCALAR_FUNCS  = Symbol[:isinf, :isnan, :isfinite, :iseven, :isodd, :isreal, :isinteger]