diff --git a/.github/workflows/c-cpp.yml b/.github/workflows/c-cpp.yml
index ba15a58..a027ec2 100644
--- a/.github/workflows/c-cpp.yml
+++ b/.github/workflows/c-cpp.yml
@@ -24,7 +24,7 @@ jobs:
     - name: install PostgreSQL
       run: |
         sudo apt update
-        sudo apt -y install postgresql-16 postgresql-client-16 postgresql-server-dev-16 libpq-dev
+        sudo apt -y install postgresql-16 postgresql-client-16 postgresql-server-dev-16 libpq-dev clang-19
     - name: make
       run: make
     - name: install
diff --git a/src/aod_sketch_c_adapter.cpp b/src/aod_sketch_c_adapter.cpp
index 48fe9f2..e93b4be 100644
--- a/src/aod_sketch_c_adapter.cpp
+++ b/src/aod_sketch_c_adapter.cpp
@@ -157,6 +157,16 @@ char* aod_sketch_to_string(const void* sketchptr, bool print_entries) {
   pg_unreachable();
 }
 
+unsigned aod_sketch_get_num_values(const void* sketchptr) {
+  try {
+    uint8_t num_values = static_cast<const compact_aod_sketch_pg*>(sketchptr)->get_num_values();
+    return num_values;
+  } catch (std::exception& e) {
+    pg_error(e.what());
+  }
+  pg_unreachable();
+}
+
 ptr_with_size aod_sketch_serialize(const void* sketchptr, unsigned header_size) {
   try {
     ptr_with_size p;
diff --git a/src/aod_sketch_c_adapter.h b/src/aod_sketch_c_adapter.h
index 7918c24..0f2032e 100644
--- a/src/aod_sketch_c_adapter.h
+++ b/src/aod_sketch_c_adapter.h
@@ -39,6 +39,7 @@ double update_aod_sketch_get_estimate(const void* sketchptr);
 double compact_aod_sketch_get_estimate(const void* sketchptr);
 void** aod_sketch_get_estimate_and_bounds(const void* sketchptr, unsigned num_std_devs);
 char* aod_sketch_to_string(const void* sketchptr, bool print_entries);
+unsigned aod_sketch_get_num_values(const void* sketchptr);
 
 struct ptr_with_size aod_sketch_serialize(const void* sketchptr, unsigned header_size);
 void* aod_sketch_deserialize(const char* buffer, unsigned length);
diff --git a/src/aod_sketch_pg_functions.c b/src/aod_sketch_pg_functions.c
index bab1c0d..a98ca1d 100644
--- a/src/aod_sketch_pg_functions.c
+++ b/src/aod_sketch_pg_functions.c
@@ -189,6 +189,10 @@ Datum pg_aod_sketch_union_agg(PG_FUNCTION_ARGS) {
 
   sketch_bytes = PG_GETARG_BYTEA_P(1);
   sketchptr = aod_sketch_deserialize(VARDATA(sketch_bytes), VARSIZE(sketch_bytes) - VARHDRSZ);
+  if (stateptr->num_values != aod_sketch_get_num_values(sketchptr)) {
+    compact_aod_sketch_delete(sketchptr);
+    elog(ERROR, "pg_aod_sketch_union_agg expects the same num_values in sketches");
+  }
   aod_union_update(stateptr->ptr, sketchptr);
   compact_aod_sketch_delete(sketchptr);