diff --git a/src/grad.jl b/src/grad.jl
index 6216cba4..c3ddc6a4 100644
--- a/src/grad.jl
+++ b/src/grad.jl
@@ -92,13 +92,16 @@ function to_vec(x::T) where {T<:LinearAlgebra.AbstractTriangular}
     return x_vec, x_vec->T(reshape(back(x_vec), size(x)))
 end
 to_vec(x::Symmetric) = vec(Matrix(x)), x_vec->Symmetric(reshape(x_vec, size(x)))
-to_vec(X::Diagonal) = vec(Matrix(X)), x_vec->Diagonal(reshape(x_vec, size(X)...))
+function to_vec(X::Diagonal)
+    diag_vec, from_diag = to_vec(X.diag)
+    return diag_vec, x_vec->Diagonal(from_diag(x_vec))
+end
 
 # Non-array data structures.
 function to_vec(x::Tuple)
     x_vecs, x_backs = zip(map(to_vec, x)...)
     sz = cumsum([map(length, x_vecs)...])
     return vcat(x_vecs...), function(v)
-        return ntuple(n->x_backs[n](v[sz[n]-length(x[n])+1:sz[n]]), length(x))
+        return ntuple(n->x_backs[n](v[sz[n]-length(x_vecs[n])+1:sz[n]]), length(x))
     end
 end
diff --git a/test/grad.jl b/test/grad.jl
index ae7c8d73..f15fefe1 100644
--- a/test/grad.jl
+++ b/test/grad.jl
@@ -54,6 +54,7 @@ using FDM: grad, jacobian, _jvp, _j′vp, jvp, j′vp, to_vec
             test_to_vec((randn(4), randn(4, 3, 2), 1))
             test_to_vec((5, randn(4, 3, 2), UpperTriangular(randn(4, 4)), 2.5))
             test_to_vec(((6, 5), 3, randn(3, 2, 0, 1)))
+            test_to_vec((Diagonal(randn(7)),))
         end
     end