diff --git a/roaringbitmap.c b/roaringbitmap.c
index 49f6b9f..ae38443 100644
--- a/roaringbitmap.c
+++ b/roaringbitmap.c
@@ -359,14 +359,14 @@ rb_or(PG_FUNCTION_ARGS) {
     if (!r1)
         ereport(ERROR,
                 (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
-                 errmsg("bitmap format is error")));
+                 errmsg("bitmap format is error: rb_or r1")));
 
     r2 = roaring_bitmap_deserialize(VARDATA(serializedbytes2));
     if (!r2) {
         roaring_bitmap_free(r1);
         ereport(ERROR,
                 (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
-                 errmsg("bitmap format is error")));
+                 errmsg("bitmap format is error: rb_or r2")));
     }
     roaring_bitmap_or_inplace(r1, r2);
     roaring_bitmap_free(r2);