diff --git a/Zend/zend_operators.c b/Zend/zend_operators.c
index 93df8cea2ea9d..d61334471f01c 100644
--- a/Zend/zend_operators.c
+++ b/Zend/zend_operators.c
@@ -2760,7 +2760,7 @@ ZEND_API zend_string* ZEND_FASTCALL zend_string_tolower_ex(zend_string *str, boo
 		if (_mm_movemask_epi8(mingle)) {
 			zend_string *res = zend_string_alloc(length, persistent);
 			memcpy(ZSTR_VAL(res), ZSTR_VAL(str), p - (unsigned char *) ZSTR_VAL(str));
-			unsigned char *q = p + (ZSTR_VAL(res) - ZSTR_VAL(str));
+			unsigned char *q = p - (unsigned char*) ZSTR_VAL(str) + (unsigned char*) ZSTR_VAL(res);
 
 			/* Lowercase the chunk we already compared. */
 			const __m128i delta = _mm_set1_epi8('a' - 'A');
@@ -2783,7 +2783,7 @@ ZEND_API zend_string* ZEND_FASTCALL zend_string_tolower_ex(zend_string *str, boo
 			zend_string *res = zend_string_alloc(length, persistent);
 			memcpy(ZSTR_VAL(res), ZSTR_VAL(str), p - (unsigned char*) ZSTR_VAL(str));
 
-			unsigned char *q = p + (ZSTR_VAL(res) - ZSTR_VAL(str));
+			unsigned char *q = p - (unsigned char*) ZSTR_VAL(str) + (unsigned char*) ZSTR_VAL(res);
 			while (p < end) {
 				*q++ = zend_tolower_ascii(*p++);
 			}
diff --git a/ext/standard/string.c b/ext/standard/string.c
index 368f0123870ce..284ad18f47f60 100644
--- a/ext/standard/string.c
+++ b/ext/standard/string.c
@@ -1373,7 +1373,7 @@ PHPAPI zend_string *php_string_toupper(zend_string *s)
 			if (c != (unsigned char*)ZSTR_VAL(s)) {
 				memcpy(ZSTR_VAL(res), ZSTR_VAL(s), c - (unsigned char*)ZSTR_VAL(s));
 			}
-			r = c + (ZSTR_VAL(res) - ZSTR_VAL(s));
+			r = c - (unsigned char*) ZSTR_VAL(s) + (unsigned char*) ZSTR_VAL(res);
 			while (c < e) {
 				*r = toupper(*c);
 				r++;
@@ -1438,7 +1438,7 @@ PHPAPI zend_string *php_string_tolower(zend_string *s)
 				if (c != (unsigned char*)ZSTR_VAL(s)) {
 					memcpy(ZSTR_VAL(res), ZSTR_VAL(s), c - (unsigned char*)ZSTR_VAL(s));
 				}
-				r = c + (ZSTR_VAL(res) - ZSTR_VAL(s));
+				r = c - (unsigned char*) ZSTR_VAL(s) + (unsigned char*) ZSTR_VAL(res);
 				while (c < e) {
 					*r = tolower(*c);
 					r++;