diff --git a/base/reducedim.jl b/base/reducedim.jl
index e74fe2b765277..0478afe1a46b6 100644
--- a/base/reducedim.jl
+++ b/base/reducedim.jl
@@ -258,8 +258,9 @@ function _mapreducedim!(f, op, R::AbstractArray, A::AbstractArrayOrBroadcasted)
         # use mapreduce_impl, which is probably better tuned to achieve higher performance
         nslices = div(length(A), lsiz)
         ibase = first(LinearIndices(A))-1
-        for i = 1:nslices
-            @inbounds R[i] = op(R[i], mapreduce_impl(f, op, A, ibase+1, ibase+lsiz))
+        for i in eachindex(R)
+            r = op(@inbounds(R[i]), mapreduce_impl(f, op, A, ibase+1, ibase+lsiz))
+            @inbounds R[i] = r
             ibase += lsiz
         end
         return R
diff --git a/test/offsetarray.jl b/test/offsetarray.jl
index 5ee918e85faf7..fb5855dfbaa0d 100644
--- a/test/offsetarray.jl
+++ b/test/offsetarray.jl
@@ -907,3 +907,10 @@ end
     v = view([1,2,3,4], :)
     @test v[Base.IdentityUnitRange(2:3)] == OffsetArray(2:3, 2:3)
 end
+
+@testset "mapreduce with OffsetRanges" begin
+    r = 5:100
+    a = OffsetArray(r, 2)
+    b = sum(a, dims=1)
+    @test b[begin] == sum(r)
+end