diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 7b03046301..5e9407c34a 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -9333,6 +9333,13 @@ 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)
 {
@@ -9348,7 +9355,7 @@ 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)));