diff --git a/src/test/modules/test_extensible/expected/test_extensible.out b/src/test/modules/test_extensible/expected/test_extensible.out
index 1fa3b6d2c55..acc0a1af7fa 100644
--- a/src/test/modules/test_extensible/expected/test_extensible.out
+++ b/src/test/modules/test_extensible/expected/test_extensible.out
@@ -91,6 +91,13 @@ SELECT test_ext_node_equal(test_ext_node_make('1234'::oid, 2),
 -- correct node type, missing field
 SELECT test_ext_node_get_relid('{EXTENSIBLENODE :extnodename TestExtNode}');
 ERROR:  unexpected end of "TestExtNode"
+-- incorrect node patterns
+SELECT test_ext_node_get_relid('{QUERY}');
+ERROR:  argument is not a serialized "TestExtNode"
+SELECT test_ext_node_get_relid('{EXTENSIBLENODE :extnodename NoSuchExtNode}');
+ERROR:  argument is not a serialized "TestExtNode"
+SELECT test_ext_node_get_relid('42');
+ERROR:  argument is not a serialized "TestExtNode"
 -- CustomScan tests
 CREATE TABLE test_extensible_tbl (id integer, val text);
 INSERT INTO test_extensible_tbl VALUES (1, 'one'), (2, 'two'), (3, 'three');
diff --git a/src/test/modules/test_extensible/sql/test_extensible.sql b/src/test/modules/test_extensible/sql/test_extensible.sql
index b23bd1cd087..72eb3cf82af 100644
--- a/src/test/modules/test_extensible/sql/test_extensible.sql
+++ b/src/test/modules/test_extensible/sql/test_extensible.sql
@@ -33,6 +33,10 @@ SELECT test_ext_node_equal(test_ext_node_make('1234'::oid, 2),
 
 -- correct node type, missing field
 SELECT test_ext_node_get_relid('{EXTENSIBLENODE :extnodename TestExtNode}');
+-- incorrect node patterns
+SELECT test_ext_node_get_relid('{QUERY}');
+SELECT test_ext_node_get_relid('{EXTENSIBLENODE :extnodename NoSuchExtNode}');
+SELECT test_ext_node_get_relid('42');
 
 -- CustomScan tests
 CREATE TABLE test_extensible_tbl (id integer, val text);
diff --git a/src/test/modules/test_extensible/test_extensible.c b/src/test/modules/test_extensible/test_extensible.c
index 2a1344f28d5..dafcd772f56 100644
--- a/src/test/modules/test_extensible/test_extensible.c
+++ b/src/test/modules/test_extensible/test_extensible.c
@@ -99,13 +99,16 @@ test_ext_node_out_cb(StringInfo str, const ExtensibleNode *node)
 }
 
 /*
- * Fetch the next token, erroring out instead of returning NULL.
+ * Fetch the next token of the string being read.
  *
- * Unlike anything in readfuncs.c, this callback is reachable with arbitrary
- * strings through SQL function calls, so we need this check.
+ * Unlike anything in readfuncs.c, this is reachable with arbitrary strings
+ * through SQL function calls, so the tokens need to be checked.
+ *
+ * If "expected" is specified, check that the token fetched matches with
+ * the input expected.  Running out of tokens is a failure.
  */
 static const char *
-test_ext_node_next_token(ReadNodeContext *ctx)
+test_ext_node_next_token(ReadNodeContext *ctx, const char *expected)
 {
 	int			length;
 	const char *token = pg_strtok(ctx, &length);
@@ -115,6 +118,14 @@ test_ext_node_next_token(ReadNodeContext *ctx)
 				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
 				 errmsg("unexpected end of \"%s\"", TEST_EXT_NODE_NAME)));
 
+	if (expected != NULL)
+	{
+		if (length != (int) strlen(expected))
+			return NULL;
+		if (memcmp(token, expected, length) != 0)
+			return NULL;
+	}
+
 	return token;
 }
 
@@ -123,11 +134,11 @@ test_ext_node_read_cb(ReadNodeContext *ctx, ExtensibleNode *node)
 {
 	TestExtNode *tnode = (TestExtNode *) node;
 
-	(void) test_ext_node_next_token(ctx);	/* skip :relid */
-	tnode->relid = atooid(test_ext_node_next_token(ctx));
+	(void) test_ext_node_next_token(ctx, NULL); /* skip :relid */
+	tnode->relid = atooid(test_ext_node_next_token(ctx, NULL));
 
-	(void) test_ext_node_next_token(ctx);	/* skip :repeat_count */
-	tnode->repeat_count = atoi(test_ext_node_next_token(ctx));
+	(void) test_ext_node_next_token(ctx, NULL); /* skip :repeat_count */
+	tnode->repeat_count = atoi(test_ext_node_next_token(ctx, NULL));
 }
 
 static const ExtensibleNodeMethods test_ext_node_methods =
@@ -469,16 +480,26 @@ test_get_custom_scan_methods(PG_FUNCTION_ARGS)
 static TestExtNode *
 text_to_test_ext_node(text *txt)
 {
-	Node	   *node = stringToNode(text_to_cstring(txt));
+	char	   *str = text_to_cstring(txt);
+	ReadNodeContext ctx;
+	TestExtNode *tnode;
 
-	if (node == NULL || !IsA(node, ExtensibleNode) ||
-		strcmp(((ExtensibleNode *) node)->extnodename, TEST_EXT_NODE_NAME) != 0)
+	ctx.str = str;
+
+	if (test_ext_node_next_token(&ctx, "{") == NULL ||
+		test_ext_node_next_token(&ctx, "EXTENSIBLENODE") == NULL ||
+		test_ext_node_next_token(&ctx, ":extnodename") == NULL ||
+		test_ext_node_next_token(&ctx, TEST_EXT_NODE_NAME) == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("argument is not a serialized \"%s\"",
 						TEST_EXT_NODE_NAME)));
 
-	return (TestExtNode *) node;
+	tnode = (TestExtNode *) stringToNode(str);
+	Assert(IsA(tnode, ExtensibleNode) &&
+		   strcmp(tnode->base.extnodename, TEST_EXT_NODE_NAME) == 0);
+
+	return tnode;
 }
 
 /*