diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index 0d04a9a0..ad993013 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -329,6 +329,20 @@ void mjCModel::CopyPlugin(std::vector<mjCPlugin*>& dest,
 }
 
 mjCModel& mjCModel::operator+=(const mjCModel& other) {
+  // TODO: use compiler settings stored in specs_ during compilation
+  std::string msg = "cannot attach mjSpecs with incompatible compiler/";
+  if (other.spec.degree != spec.degree) {
+    throw mjCError(nullptr, (msg + "angle attribute").c_str());
+  }
+  if (other.spec.autolimits != spec.autolimits) {
+    throw mjCError(nullptr, (msg + "autolimits attribute").c_str());
+  }
+  if (other.spec.eulerseq[0] != spec.eulerseq[0] ||
+      other.spec.eulerseq[1] != spec.eulerseq[1] ||
+      other.spec.eulerseq[2] != spec.eulerseq[2]) {
+    throw mjCError(nullptr, (msg + "eulerseq attribute").c_str());
+  }
+
   // create global lists
   mjCBody *world = bodies_[0];
   if (compiled) {
diff --git a/test/user/user_api_test.cc b/test/user/user_api_test.cc
index 42123755..be4b5279 100644
--- a/test/user/user_api_test.cc
+++ b/test/user/user_api_test.cc
@@ -37,6 +37,7 @@ namespace {
 
 using ::testing::HasSubstr;
 using ::testing::NotNull;
+using ::testing::IsNull;
 
 
 // -------------------------- test model manipulation  -------------------------
@@ -1486,5 +1487,24 @@ TEST_F(MujocoTest, RepeatedAttachKeyframe) {
     mj_deleteModel(model_2);
 }
 
+TEST_F(MujocoTest, DifferentUnitsNotAllowed) {
+  mjSpec* spec_1 = mj_makeSpec();
+  mjSpec* spec_2 = mj_makeSpec();
+  spec_1->degree = 1;
+  spec_2->degree = 0;
+
+  mjsBody* body = mjs_addBody(mjs_findBody(spec_1, "world"), 0);
+  mjsFrame* frame = mjs_addFrame(mjs_findBody(spec_2, "world"), 0);
+
+  constexpr char msg[] = "mjSpecs with incompatible compiler/angle";
+  EXPECT_THAT(mjs_attachBody(frame, body, "child-", ""), IsNull());
+  EXPECT_THAT(mjs_attachFrame(body, frame, "child-", ""), IsNull());
+  EXPECT_THAT(mjs_getError(spec_1), HasSubstr(msg));
+  EXPECT_THAT(mjs_getError(spec_2), HasSubstr(msg));
+
+  mj_deleteSpec(spec_1);
+  mj_deleteSpec(spec_2);
+}
+
 }  // namespace
 }  // namespace mujoco