diff --git a/contrib/auth_delay/auth_delay.c b/contrib/auth_delay/auth_delay.c
index 5820ac328d..d11dd1e416 100644
--- a/contrib/auth_delay/auth_delay.c
+++ b/contrib/auth_delay/auth_delay.c
@@ -67,6 +67,9 @@ _PG_init(void)
 							NULL,
 							NULL,
 							NULL);
+
+	EmitWarningsOnPlaceholders("auth_delay");
+
 	/* Install Hooks */
 	original_client_auth_hook = ClientAuthentication_hook;
 	ClientAuthentication_hook = auth_delay_checks;
diff --git a/contrib/pg_trgm/trgm_op.c b/contrib/pg_trgm/trgm_op.c
index fb38135f7a..0407c7dd64 100644
--- a/contrib/pg_trgm/trgm_op.c
+++ b/contrib/pg_trgm/trgm_op.c
@@ -100,6 +100,8 @@ _PG_init(void)
 							 NULL,
 							 NULL,
 							 NULL);
+
+	EmitWarningsOnPlaceholders("pg_trgm");
 }
 
 /*
diff --git a/contrib/postgres_fdw/option.c b/contrib/postgres_fdw/option.c
index 48c7417e6e..36555398ec 100644
--- a/contrib/postgres_fdw/option.c
+++ b/contrib/postgres_fdw/option.c
@@ -469,4 +469,6 @@ _PG_init(void)
 							   NULL,
 							   NULL,
 							   NULL);
+
+	EmitWarningsOnPlaceholders("postgres_fdw");
 }
diff --git a/contrib/sepgsql/hooks.c b/contrib/sepgsql/hooks.c
index 19a3ffb7ff..44a09c35a7 100644
--- a/contrib/sepgsql/hooks.c
+++ b/contrib/sepgsql/hooks.c
@@ -455,6 +455,8 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
+	EmitWarningsOnPlaceholders("sepgsql");
+
 	/* Initialize userspace access vector cache */
 	sepgsql_avc_init();
 
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index e91d5a3cfd..07647a0d84 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -9322,6 +9322,13 @@ DefineCustomEnumVariable(const char *name,
 	define_custom_variable(&var->gen);
 }
 
+/*
+ * For historical context, this function name is EmitWarningsOnPlaceholders,
+ * but it emits an error when an invalid custom GUC is set.
+ *
+ * If we use DefineCustomXXXVariable, it is recommended that this function is
+ * added.
+ */
 void
 EmitWarningsOnPlaceholders(const char *className)
 {
@@ -9336,7 +9343,7 @@ EmitWarningsOnPlaceholders(const char *className)
 			strncmp(className, var->name, classLen) == 0 &&
 			var->name[classLen] == GUC_QUALIFIER_SEPARATOR)
 		{
-			ereport(WARNING,
+			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_OBJECT),
 					 errmsg("unrecognized configuration parameter \"%s\"",
 							var->name)));
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index e11837559d..1389aa0943 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -474,6 +474,8 @@ _PG_init(void)
 							   PGC_SUSET, 0,
 							   NULL, NULL, NULL);
 
+	EmitWarningsOnPlaceholders("pltclu");
+
 	pltcl_pm_init_done = true;
 }
 
diff --git a/src/test/modules/delay_execution/delay_execution.c b/src/test/modules/delay_execution/delay_execution.c
index b3d0841ba8..8ec623ac52 100644
--- a/src/test/modules/delay_execution/delay_execution.c
+++ b/src/test/modules/delay_execution/delay_execution.c
@@ -91,6 +91,8 @@ _PG_init(void)
 							NULL,
 							NULL);
 
+	EmitWarningsOnPlaceholders("delay_execution");
+
 	/* Install our hook */
 	prev_planner_hook = planner_hook;
 	planner_hook = delay_execution_planner;
diff --git a/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c b/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
index 6b0a3db104..3ba33e501c 100644
--- a/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
+++ b/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
@@ -48,6 +48,9 @@ _PG_init(void)
 							   NULL,
 							   NULL,
 							   NULL);
+
+	EmitWarningsOnPlaceholders("ssl_passphrase");
+
 	if (ssl_passphrase)
 		openssl_tls_init_hook = set_rot13;
 }
diff --git a/src/test/modules/worker_spi/worker_spi.c b/src/test/modules/worker_spi/worker_spi.c
index 0b6246676b..adb02d8cb8 100644
--- a/src/test/modules/worker_spi/worker_spi.c
+++ b/src/test/modules/worker_spi/worker_spi.c
@@ -322,6 +322,8 @@ _PG_init(void)
 							   0,
 							   NULL, NULL, NULL);
 
+	EmitWarningsOnPlaceholders("worker_spi");
+
 	/* set up common data for all our workers */
 	memset(&worker, 0, sizeof(worker));
 	worker.bgw_flags = BGWORKER_SHMEM_ACCESS |