diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index e8a068b4..c2fff3d8 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -4064,7 +4064,7 @@ void mjCKey::Compile(const mjModel* m) {
       qpos[i] = (double)m->qpos0[i];
     }
   } else if (qpos.size()!=m->nq) {
-    throw mjCError(this, "key %d: invalid qpos size", 0, id);
+    throw mjCError(this, "key %d: invalid qpos size, expected length %d", nullptr, id, m->nq);
   }
 
   // qvel: allocate or check size
@@ -4074,7 +4074,7 @@ void mjCKey::Compile(const mjModel* m) {
       qvel[i] = 0;
     }
   } else if (qvel.size()!=m->nv) {
-    throw mjCError(this, "key %d: invalid qvel size", 0, id);
+    throw mjCError(this, "key %d: invalid qvel size, expected length %d", nullptr, id, m->nv);
   }
 
   // act: allocate or check size
@@ -4084,7 +4084,7 @@ void mjCKey::Compile(const mjModel* m) {
       act[i] = 0;
     }
   } else if (act.size()!=m->na) {
-    throw mjCError(this, "key %d: invalid act size", 0, id);
+    throw mjCError(this, "key %d: invalid act size, expected length %d", nullptr, id, m->na);
   }
 
   // mpos: allocate or check size
@@ -4101,7 +4101,7 @@ void mjCKey::Compile(const mjModel* m) {
       }
     }
   } else if (mpos.size()!=3*m->nmocap) {
-    throw mjCError(this, "key %d: invalid mpos size", 0, id);
+    throw mjCError(this, "key %d: invalid mpos size, expected length %d", nullptr, id, 3*m->nmocap);
   }
 
   // mquat: allocate or check size
@@ -4119,7 +4119,7 @@ void mjCKey::Compile(const mjModel* m) {
       }
     }
   } else if (mquat.size()!=4*m->nmocap) {
-    throw mjCError(this, "key %d: invalid mquat size", 0, id);
+    throw mjCError(this, "key %d: invalid mquat size, expected length %d", nullptr, id, 4*m->nmocap);
   }
 
   // ctrl: allocate or check size
@@ -4129,7 +4129,7 @@ void mjCKey::Compile(const mjModel* m) {
       ctrl[i] = 0;
     }
   } else if (ctrl.size()!=m->nu) {
-    throw mjCError(this, "key %d: invalid ctrl size", 0, id);
+    throw mjCError(this, "key %d: invalid ctrl size, expected length %d", nullptr, id, m->nu);
   }
 
 }
diff --git a/test/user/user_objects_test.cc b/test/user/user_objects_test.cc
index d9631aec..e0c6256b 100644
--- a/test/user/user_objects_test.cc
+++ b/test/user/user_objects_test.cc
@@ -39,9 +39,11 @@ using ::testing::NotNull;
 
 // ------------------------ test keyframes -------------------------------------
 
+using KeyframeTest = MujocoTest;
+
 constexpr char kKeyframePath[] = "user/testdata/keyframe.xml";
 
-TEST_F(MujocoTest, KeyFrameTest) {
+TEST_F(KeyframeTest, CheckValues) {
   const std::string xml_path = GetTestDataFilePath(kKeyframePath);
   mjModel* model = mj_loadXML(xml_path.c_str(), nullptr, nullptr, 0);
   ASSERT_THAT(model, NotNull());
@@ -59,7 +61,7 @@ TEST_F(MujocoTest, KeyFrameTest) {
   mj_deleteModel(model);
 }
 
-TEST_F(MujocoTest, ResetDataKeyframeTest) {
+TEST_F(KeyframeTest, ResetDataKeyframe) {
   const std::string xml_path = GetTestDataFilePath(kKeyframePath);
   mjModel* model = mj_loadXML(xml_path.c_str(), nullptr, nullptr, 0);
   ASSERT_THAT(model, NotNull());
@@ -91,6 +93,21 @@ TEST_F(MujocoTest, ResetDataKeyframeTest) {
   mj_deleteModel(model);
 }
 
+TEST_F(KeyframeTest, BadSize) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <keyframe>
+      <key qpos="1"/>
+    </keyframe>
+  </mujoco>
+  )";
+  char error[1024];
+  size_t error_sz = 1024;
+  mjModel* model = LoadModelFromString(xml, error, error_sz);
+  EXPECT_THAT(model, ::testing::IsNull());
+  EXPECT_THAT(error, HasSubstr("invalid qpos size, expected length 0"));
+}
+
 // ------------- test relative frame sensor compilation-------------------------
 
 using RelativeFrameSensorParsingTest = MujocoTest;