diff --git a/src/Common/UTF8Helpers.h b/src/Common/UTF8Helpers.h
index 926bb6a7f92..129a745afe2 100644
--- a/src/Common/UTF8Helpers.h
+++ b/src/Common/UTF8Helpers.h
@@ -42,7 +42,7 @@ inline void syncForward(const UInt8 * & s, const UInt8 * const end)
 /// returns UTF-8 code point sequence length judging by it's first octet
 inline size_t seqLength(const UInt8 first_octet)
 {
-    if (first_octet < 0x80u)
+    if (first_octet < 0x80 || first_octet >= 0xF8)  /// The specs of UTF-8.
         return 1;
 
     const size_t bits = 8;
diff --git a/tests/queries/0_stateless/01431_utf8_ubsan.reference b/tests/queries/0_stateless/01431_utf8_ubsan.reference
new file mode 100644
index 00000000000..c98c950d535
--- /dev/null
+++ b/tests/queries/0_stateless/01431_utf8_ubsan.reference
@@ -0,0 +1,2 @@
+FF
+FF
diff --git a/tests/queries/0_stateless/01431_utf8_ubsan.sql b/tests/queries/0_stateless/01431_utf8_ubsan.sql
new file mode 100644
index 00000000000..d6a299225b1
--- /dev/null
+++ b/tests/queries/0_stateless/01431_utf8_ubsan.sql
@@ -0,0 +1,2 @@
+SELECT hex(lowerUTF8('\xFF'));
+SELECT hex(upperUTF8('\xFF'));