diff --git a/m-string.h b/m-string.h
index 8f80802c..2f3cd60b 100644
--- a/m-string.h
+++ b/m-string.h
@@ -1325,7 +1325,7 @@ stringi_utf8_decode(char c, stringi_utf8_state_e *state,
                     string_unicode_t *unicode)
 {
   const unsigned int type = m_core_clz32((unsigned char)~c) - (sizeof(uint32_t) - 1) * CHAR_BIT;
-  const string_unicode_t mask1 = -(string_unicode_t)(*state != STRINGI_UTF8_STARTING);
+  const string_unicode_t mask1 = (UINT_MAX - (string_unicode_t)(*state != STRINGI_UTF8_STARTING) + 1);
   const string_unicode_t mask2 = (0xFFU >> type);
   *unicode = ((*unicode << 6) & mask1) | ((unsigned int) c & mask2);
   *state = (stringi_utf8_state_e) STRINGI_UTF8_STATE_TAB[(unsigned int) *state + type];