diff --git a/expected/hypopg.out b/expected/hypopg.out
index 990cb46..a00648f 100644
--- a/expected/hypopg.out
+++ b/expected/hypopg.out
@@ -105,6 +105,9 @@ ORDER BY indexrelid;
  f
 (3 rows)
 
+-- Should detect invalid argument
+SELECT hypopg_relation_size(1);
+ERROR:  oid 1 is not a hypothetical index
 -- locally disable hypoopg
 SET hypopg.enabled to false;
 -- no hypothetical index should be used
diff --git a/hypopg_index.c b/hypopg_index.c
index 204fc3e..e7a0b7a 100644
--- a/hypopg_index.c
+++ b/hypopg_index.c
@@ -1388,6 +1388,7 @@ hypopg_relation_size(PG_FUNCTION_ARGS)
 	double		tuples;
 	Oid			indexid = PG_GETARG_OID(0);
 	ListCell   *lc;
+	bool		found = false;
 
 	pages = 0;
 	tuples = 0;
@@ -1398,9 +1399,14 @@ hypopg_relation_size(PG_FUNCTION_ARGS)
 		if (entry->oid == indexid)
 		{
 			hypo_estimate_index_simple(entry, &pages, &tuples);
+			found = true;
+			break;
 		}
 	}
 
+	if (!found)
+		elog(ERROR, "oid %u is not a hypothetical index", indexid);
+
 	PG_RETURN_INT64(pages * 1.0L * BLCKSZ);
 }
 
diff --git a/test/sql/hypopg.sql b/test/sql/hypopg.sql
index 28e3fcb..64cfcd5 100644
--- a/test/sql/hypopg.sql
+++ b/test/sql/hypopg.sql
@@ -70,6 +70,9 @@ SELECT hypopg_relation_size(indexrelid) = current_setting('block_size')::bigint
 FROM hypopg()
 ORDER BY indexrelid;
 
+-- Should detect invalid argument
+SELECT hypopg_relation_size(1);
+
 -- locally disable hypoopg
 SET hypopg.enabled to false;