diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7939bb0..7cb989a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,6 +33,16 @@ jobs:
           version: ${{ matrix.version }}
           arch: ${{ matrix.arch }}
       - uses: julia-actions/cache@v2
+      - uses: actions/checkout@v4
+        with:
+          repository: AayushSabharwal/MultivariatePolynomials.jl
+          ref: as/poly-merge-nonconcrete
+          path: upstream
+      - name: Use branch
+        shell: julia --color=yes --project=@. {0}
+        run: |
+          using Pkg
+          Pkg.develop(PackageSpec(; path = "upstream"))
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
         with:
diff --git a/src/operators.jl b/src/operators.jl
index dd27226..cbe67b7 100644
--- a/src/operators.jl
+++ b/src/operators.jl
@@ -183,9 +183,9 @@ end
 # TODO need to check that this also works for non-commutative
 function MA.operate!(
     op::Union{typeof(+),typeof(-)},
-    p::Polynomial{V},
-    q::Polynomial{V},
-) where {V<:Commutative}
+    p::Polynomial{V, M1, T1},
+    q::Polynomial{V, M2, T2},
+) where {V<:Commutative, M1, M2, T1, T2}
     if MP.variables(p) != MP.variables(q)
         allvars, maps = ___add_variables!(p, q)
         if length(allvars) == length(MP.variables(q))
@@ -239,6 +239,7 @@ function MA.operate!(
         combine,
         keep,
         resize,
+        Tuple{MA.promote_operation(op, T1, T2), Vector{Int}},
     )
     return p
 end
diff --git a/test/mutable_arithmetics.jl b/test/mutable_arithmetics.jl
index 5529013..f5959d7 100644
--- a/test/mutable_arithmetics.jl
+++ b/test/mutable_arithmetics.jl
@@ -2,6 +2,7 @@ using Test
 
 import MutableArithmetics
 const MA = MutableArithmetics
+import MultivariatePolynomials as MP
 
 using DynamicPolynomials
 
@@ -100,3 +101,15 @@ end
         @test (@allocated MA.operate!(-, poly2, x)) <= 144
     end
 end
+
+@testset "Non-concrete in-place polynomial addition" begin
+    @polyvar p q r s
+    p1 = MP.polynomial(r - q, Number) + MP.polynomial(3//5 * p^2, Number)
+    p2 = MP.polynomial(1//2 + s, Number) + MP.polynomial(p^2, Number)
+    result = p1 + p2
+    @test isequal(result, MA.operate!(+, p1, p2))
+
+    p1 = MP.polynomial(r - q, Number) + MP.polynomial(3//5 * p^2, Number)
+    p2 = MP.polynomial(1//2 + s, Number) + MP.polynomial(p^2, Number)
+    @test isequal(result, MA.operate!(+, p2, p1))
+end