diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index bff949a40b..f345eceb5b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -148,6 +148,8 @@ extern bool optimize_bounded_sort;
 
 static int	GUC_check_errcode_value;
 
+static List *reserved_class_prefix = NIL;
+
 /* global variables for check hook support */
 char	   *GUC_check_errmsg_string;
 char	   *GUC_check_errdetail_string;
@@ -235,8 +237,6 @@ static bool check_recovery_target_lsn(char **newval, void **extra, GucSource sou
 static void assign_recovery_target_lsn(const char *newval, void *extra);
 static bool check_primary_slot_name(char **newval, void **extra, GucSource source);
 static bool check_default_with_oids(bool *newval, void **extra, GucSource source);
-static void check_reserved_prefixes(const char *varName);
-static List *reserved_class_prefix = NIL;
 
 /* Private functions in guc-file.l that need to be called from guc.c */
 static ConfigVariable *ProcessConfigFileInternal(GucContext context,
@@ -5569,18 +5569,44 @@ find_option(const char *name, bool create_placeholders, bool skip_errors,
 		 * doesn't contain a separator, don't assume that it was meant to be a
 		 * placeholder.
 		 */
-		if (strchr(name, GUC_QUALIFIER_SEPARATOR) != NULL)
+		const char *sep = strchr(name, GUC_QUALIFIER_SEPARATOR);
+
+		if (sep != NULL)
 		{
-			if (valid_custom_variable_name(name))
-				return add_placeholder_variable(name, elevel);
-			/* A special error message seems desirable here */
-			if (!skip_errors)
-				ereport(elevel,
-						(errcode(ERRCODE_INVALID_NAME),
-						 errmsg("invalid configuration parameter name \"%s\"",
-								name),
-						 errdetail("Custom parameter names must be two or more simple identifiers separated by dots.")));
-			return NULL;
+			size_t		classLen = sep - name;
+			ListCell   *lc;
+
+			/* The name must be syntactically acceptable ... */
+			if (!valid_custom_variable_name(name))
+			{
+				if (!skip_errors)
+					ereport(elevel,
+							(errcode(ERRCODE_INVALID_NAME),
+							 errmsg("invalid configuration parameter name \"%s\"",
+									name),
+							 errdetail("Custom parameter names must be two or more simple identifiers separated by dots.")));
+				return NULL;
+			}
+			/* ... and it must not match any previously-reserved prefix */
+			foreach(lc, reserved_class_prefix)
+			{
+				const char *rcprefix = lfirst(lc);
+
+				if (strlen(rcprefix) == classLen &&
+					strncmp(name, rcprefix, classLen) == 0)
+				{
+					if (!skip_errors)
+						ereport(elevel,
+								(errcode(ERRCODE_INVALID_NAME),
+								 errmsg("invalid configuration parameter name \"%s\"",
+										name),
+								 errdetail("\"%s\" is a reserved prefix.",
+										   rcprefix)));
+					return NULL;
+				}
+			}
+			/* OK, create it */
+			return add_placeholder_variable(name, elevel);
 		}
 	}
 
@@ -8764,7 +8790,6 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 									 (superuser() ? PGC_SUSET : PGC_USERSET),
 									 PGC_S_SESSION,
 									 action, true, 0, false);
-			check_reserved_prefixes(stmt->name);
 			break;
 		case VAR_SET_MULTI:
 
@@ -8850,8 +8875,6 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 									 (superuser() ? PGC_SUSET : PGC_USERSET),
 									 PGC_S_SESSION,
 									 action, true, 0, false);
-
-			check_reserved_prefixes(stmt->name);
 			break;
 		case VAR_RESET_ALL:
 			ResetAllOptions();
@@ -9345,8 +9368,9 @@ EmitWarningsOnPlaceholders(const char *className)
 {
 	int			classLen = strlen(className);
 	int			i;
-	MemoryContext	oldcontext;
+	MemoryContext oldcontext;
 
+	/* Check for existing placeholders. */
 	for (i = 0; i < num_guc_variables; i++)
 	{
 		struct config_generic *var = guc_variables[i];
@@ -9362,48 +9386,12 @@ EmitWarningsOnPlaceholders(const char *className)
 		}
 	}
 
+	/* And remember the name so we can prevent future mistakes. */
 	oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 	reserved_class_prefix = lappend(reserved_class_prefix, pstrdup(className));
 	MemoryContextSwitchTo(oldcontext);
 }
 
-/*
- * Check a setting name against prefixes previously reserved by
- * EmitWarningsOnPlaceholders() and throw a warning if matching.
- */
-static void
-check_reserved_prefixes(const char *varName)
-{
-	char	   *sep = strchr(varName, GUC_QUALIFIER_SEPARATOR);
-
-	if (sep)
-	{
-		size_t		classLen = sep - varName;
-		ListCell   *lc;
-
-		foreach(lc, reserved_class_prefix)
-		{
-			char	   *rcprefix = lfirst(lc);
-
-			if (strncmp(varName, rcprefix, classLen) == 0)
-			{
-				for (int i = 0; i < num_guc_variables; i++)
-				{
-					struct config_generic *var = guc_variables[i];
-
-					if ((var->flags & GUC_CUSTOM_PLACEHOLDER) != 0 &&
-						strcmp(varName, var->name) == 0)
-					{
-						ereport(WARNING,
-								(errcode(ERRCODE_UNDEFINED_OBJECT),
-								 errmsg("unrecognized configuration parameter \"%s\"", var->name),
-								 errdetail("\"%.*s\" is a reserved prefix.", (int) classLen, var->name)));
-					}
-				}
-			}
-		}
-	}
-}
 
 /*
  * SHOW command
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 2433038c3e..c50f3fe817 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -818,17 +818,11 @@ SET foo = false;  -- no such setting
 ERROR:  unrecognized configuration parameter "foo"
 -- test setting a parameter with a registered prefix (plpgsql)
 SET plpgsql.extra_foo_warnings = false;  -- no such setting
-WARNING:  unrecognized configuration parameter "plpgsql.extra_foo_warnings"
+ERROR:  invalid configuration parameter name "plpgsql.extra_foo_warnings"
 DETAIL:  "plpgsql" is a reserved prefix.
-SHOW plpgsql.extra_foo_warnings;  -- but the parameter is set
- plpgsql.extra_foo_warnings 
-----------------------------
- false
-(1 row)
-
 -- cleanup
 RESET foo;
 ERROR:  unrecognized configuration parameter "foo"
 RESET plpgsql.extra_foo_warnings;
-WARNING:  unrecognized configuration parameter "plpgsql.extra_foo_warnings"
+ERROR:  invalid configuration parameter name "plpgsql.extra_foo_warnings"
 DETAIL:  "plpgsql" is a reserved prefix.
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index b57758ed27..2f89974d8c 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -317,7 +317,6 @@ SET foo = false;  -- no such setting
 
 -- test setting a parameter with a registered prefix (plpgsql)
 SET plpgsql.extra_foo_warnings = false;  -- no such setting
-SHOW plpgsql.extra_foo_warnings;  -- but the parameter is set
 
 -- cleanup
 RESET foo;