diff --git a/src/backend/utils/adt/oracle_compat.c b/src/backend/utils/adt/oracle_compat.c
index 471c6967be0..87deeda7b23 100644
--- a/src/backend/utils/adt/oracle_compat.c
+++ b/src/backend/utils/adt/oracle_compat.c
@@ -932,8 +932,10 @@ ascii(PG_FUNCTION_ARGS)
 	text	   *string = PG_GETARG_TEXT_PP(0);
 	int			encoding = GetDatabaseEncoding();
 	unsigned char *data;
+	int			len;
 
-	if (VARSIZE_ANY_EXHDR(string) <= 0)
+	len = VARSIZE_ANY_EXHDR(string);
+	if (len <= 0)
 		PG_RETURN_INT32(0);
 
 	data = (unsigned char *) VARDATA_ANY(string);
@@ -956,18 +958,31 @@ ascii(PG_FUNCTION_ARGS)
 			result = *data & 0x0F;
 			tbytes = 2;
 		}
-		else
+		else if (*data > 0xC0)
 		{
-			Assert(*data > 0xC0);
 			result = *data & 0x1f;
 			tbytes = 1;
 		}
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_CHARACTER_NOT_IN_REPERTOIRE),
+					 errmsg("invalid byte sequence for encoding \"%s\"",
+							GetDatabaseEncodingName())));
 
-		Assert(tbytes > 0);
+		/* All continuation bytes are present in the input */
+		if (tbytes >= len)
+			ereport(ERROR,
+					(errcode(ERRCODE_CHARACTER_NOT_IN_REPERTOIRE),
+					 errmsg("invalid byte sequence for encoding \"%s\"",
+							GetDatabaseEncodingName())));
 
 		for (i = 1; i <= tbytes; i++)
 		{
-			Assert((data[i] & 0xC0) == 0x80);
+			if (unlikely((data[i] & 0xC0) != 0x80))
+				ereport(ERROR,
+						(errcode(ERRCODE_CHARACTER_NOT_IN_REPERTOIRE),
+						 errmsg("invalid byte sequence for encoding \"%s\"",
+								GetDatabaseEncodingName())));
 			result = (result << 6) + (data[i] & 0x3f);
 		}
 
diff --git a/src/test/regress/input/encoding.source b/src/test/regress/input/encoding.source
index d080bef2f80..72652f5494c 100644
--- a/src/test/regress/input/encoding.source
+++ b/src/test/regress/input/encoding.source
@@ -223,6 +223,18 @@ ALTER TABLE toast_3b_utf8 RENAME TO toast_4b_utf8;
 UPDATE toast_4b_utf8 SET c = repeat(U&'\+01F680', 3000);
 SELECT SUBSTRING(c FROM 3000 FOR 1) FROM toast_4b_utf8;
 
+-- ascii() and multibyte characters
+SELECT ascii(test_bytea_to_text('\xc3'));
+SELECT ascii(test_bytea_to_text('\xe2'));
+SELECT ascii(test_bytea_to_text('\xe282'));
+SELECT ascii(test_bytea_to_text('\xf0'));
+SELECT ascii(test_bytea_to_text('\xf09f'));
+SELECT ascii(test_bytea_to_text('\xf09f98'));
+-- invalid continuation byte
+SELECT ascii(test_bytea_to_text('\xc3ff'));
+-- invalid leading byte
+SELECT ascii(test_bytea_to_text('\x80'));
+
 DROP TABLE encoding_tests;
 DROP TABLE toast_4b_utf8;
 DROP FUNCTION test_encoding;
diff --git a/src/test/regress/output/encoding.source b/src/test/regress/output/encoding.source
index 6e730e8737e..af5c21cb59a 100644
--- a/src/test/regress/output/encoding.source
+++ b/src/test/regress/output/encoding.source
@@ -413,6 +413,25 @@ SELECT SUBSTRING(c FROM 3000 FOR 1) FROM toast_4b_utf8;
  🚀
 (1 row)
 
+-- ascii() and multibyte characters
+SELECT ascii(test_bytea_to_text('\xc3'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+SELECT ascii(test_bytea_to_text('\xe2'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+SELECT ascii(test_bytea_to_text('\xe282'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+SELECT ascii(test_bytea_to_text('\xf0'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+SELECT ascii(test_bytea_to_text('\xf09f'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+SELECT ascii(test_bytea_to_text('\xf09f98'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+-- invalid continuation byte
+SELECT ascii(test_bytea_to_text('\xc3ff'));
+ERROR:  invalid byte sequence for encoding "UTF8"
+-- invalid leading byte
+SELECT ascii(test_bytea_to_text('\x80'));
+ERROR:  invalid byte sequence for encoding "UTF8"
 DROP TABLE encoding_tests;
 DROP TABLE toast_4b_utf8;
 DROP FUNCTION test_encoding;