diff --git a/src/backend/utils/adt/pg_locale_builtin.c b/src/backend/utils/adt/pg_locale_builtin.c
index 01d4f55b07e..7c36fd5091b 100644
--- a/src/backend/utils/adt/pg_locale_builtin.c
+++ b/src/backend/utils/adt/pg_locale_builtin.c
@@ -62,21 +62,33 @@ initcap_wbnext(void *state)
 
 	while (wbstate->offset < wbstate->len)
 	{
-		char32_t	u = utf8_to_unicode((const unsigned char *) wbstate->str +
+		int			ulen = pg_utf_mblen((const unsigned char *) wbstate->str +
 										wbstate->offset);
-		bool		curr_alnum = pg_u_isalnum(u, wbstate->posix);
+		char32_t	u;
+		bool		curr_alnum;
+		size_t		prev_offset = wbstate->offset;
+
+		/* invalid UTF8 */
+		if (wbstate->offset + ulen > wbstate->len)
+		{
+			wbstate->init = true;
+			wbstate->offset = wbstate->len;
+			return prev_offset;
+		}
+
+		u = utf8_to_unicode((const unsigned char *) wbstate->str +
+							wbstate->offset);
+		curr_alnum = pg_u_isalnum(u, wbstate->posix);
 
 		if (!wbstate->init || curr_alnum != wbstate->prev_alnum)
 		{
-			size_t		prev_offset = wbstate->offset;
-
 			wbstate->init = true;
-			wbstate->offset += unicode_utf8len(u);
+			wbstate->offset += ulen;
 			wbstate->prev_alnum = curr_alnum;
 			return prev_offset;
 		}
 
-		wbstate->offset += unicode_utf8len(u);
+		wbstate->offset += ulen;
 	}
 
 	return wbstate->len;
diff --git a/src/common/unicode/case_test.c b/src/common/unicode/case_test.c
index a0dbf00b671..31ea94513bf 100644
--- a/src/common/unicode/case_test.c
+++ b/src/common/unicode/case_test.c
@@ -296,6 +296,8 @@ tfunc_fold(char *dst, size_t dstsize, const char *src,
 static void
 test_convert_case(void)
 {
+	size_t		needed;
+
 	/* test string with no case changes */
 	test_convert(tfunc_lower, "√∞", "√∞");
 	/* test adjust-to-cased behavior */
@@ -320,6 +322,12 @@ test_convert_case(void)
 	/* U+FF11 FULLWIDTH ONE is alphanumeric for full case mapping */
 	test_convert(tfunc_title, "\uFF11a", "\uFF11a");
 
+	/* invalid UTF8: truncated multibyte sequence */
+	needed = unicode_strfold(NULL, 0, "abc\xCE", 4, false);
+	Assert(needed == 3);
+	/* invalid UTF8: invalid byte */
+	needed = unicode_strfold(NULL, 0, "abc\xF8xyz", 7, false);
+	Assert(needed == 3);
 
 #ifdef USE_ICU
 	icu_test_full("");
diff --git a/src/common/unicode_case.c b/src/common/unicode_case.c
index d6ee00b7d9c..42eb7d22211 100644
--- a/src/common/unicode_case.c
+++ b/src/common/unicode_case.c
@@ -189,6 +189,22 @@ unicode_strfold(char *dst, size_t dstsize, const char *src, size_t srclen,
 						NULL);
 }
 
+/* local version of pg_utf_mblen() to be inlinable */
+static int
+utf8_mblen(const unsigned char *s)
+{
+	if ((*s & 0x80) == 0)
+		return 1;
+	else if ((*s & 0xe0) == 0xc0)
+		return 2;
+	else if ((*s & 0xf0) == 0xe0)
+		return 3;
+	else if ((*s & 0xf8) == 0xf0)
+		return 4;
+	else
+		return -1;
+}
+
 /*
  * Implement Unicode Default Case Conversion algorithm.
  *
@@ -227,12 +243,18 @@ convert_case(char *dst, size_t dstsize, const char *src, size_t srclen,
 
 	while (srcoff < srclen)
 	{
-		char32_t	u1 = utf8_to_unicode((const unsigned char *) src + srcoff);
-		int			u1len = unicode_utf8len(u1);
+		int			u1len = utf8_mblen((const unsigned char *) src + srcoff);
+		char32_t	u1;
 		char32_t	simple = 0;
 		const char32_t *special = NULL;
 		enum CaseMapResult casemap_result;
 
+		/* invalid UTF8 */
+		if (u1len < 0 || srcoff + u1len > srclen)
+			break;
+
+		u1 = utf8_to_unicode((const unsigned char *) src + srcoff);
+
 		if (str_casekind == CaseTitle)
 		{
 			if (srcoff == boundary)
@@ -316,7 +338,14 @@ check_final_sigma(const unsigned char *str, size_t len, size_t offset)
 	{
 		if ((str[i] & 0x80) == 0 || (str[i] & 0xC0) == 0xC0)
 		{
-			char32_t	curr = utf8_to_unicode(str + i);
+			int			u1len = utf8_mblen((const unsigned char *) str + i);
+			char32_t	curr;
+
+			/* invalid UTF8 */
+			if (u1len < 0 || i + u1len > len)
+				return false;
+
+			curr = utf8_to_unicode(str + i);
 
 			if (pg_u_prop_case_ignorable(curr))
 				continue;
@@ -327,8 +356,8 @@ check_final_sigma(const unsigned char *str, size_t len, size_t offset)
 		}
 		else if ((str[i] & 0xC0) == 0x80)
 			continue;
-
-		Assert(false);			/* invalid UTF-8 */
+		else
+			return false;			/* invalid UTF8 */
 	}
 
 	/* end of string is not followed by a Cased character */
@@ -340,7 +369,14 @@ check_final_sigma(const unsigned char *str, size_t len, size_t offset)
 	{
 		if ((str[i] & 0x80) == 0 || (str[i] & 0xC0) == 0xC0)
 		{
-			char32_t	curr = utf8_to_unicode(str + i);
+			int			u1len = utf8_mblen((const unsigned char *) str + i);
+			char32_t	curr;
+
+			/* invalid UTF8 */
+			if (u1len < 0 || i + u1len > len)
+				return false;
+
+			curr = utf8_to_unicode(str + i);
 
 			if (pg_u_prop_case_ignorable(curr))
 				continue;
@@ -351,8 +387,8 @@ check_final_sigma(const unsigned char *str, size_t len, size_t offset)
 		}
 		else if ((str[i] & 0xC0) == 0x80)
 			continue;
-
-		Assert(false);			/* invalid UTF-8 */
+		else
+			return false;			/* invalid UTF8 */
 	}
 
 	return true;