diff --git a/Zend/zend_execute.c b/Zend/zend_execute.c
index d72fc7369aff7..62c4fd4b9265a 100644
--- a/Zend/zend_execute.c
+++ b/Zend/zend_execute.c
@@ -628,6 +628,17 @@ static inline int zend_verify_arg_type(zend_function *zf, zend_uint arg_num, zva
 		}
 	} else if (cur_arg_info->type_hint) {
 		switch(cur_arg_info->type_hint) {
+			case IS_LONG:
+				if (!arg) {
+					return zend_verify_arg_error(E_RECOVERABLE_ERROR, zf, arg_num, "be of the type long", "", "none", "" TSRMLS_CC);
+				}
+
+				if (Z_TYPE_P(arg) != IS_LONG && (Z_TYPE_P(arg) != IS_NULL || !cur_arg_info->allow_null)
+						&& (Z_TYPE_P(arg) != IS_STRING || is_numeric_string(Z_STRVAL_P(arg), Z_STRLEN_P(arg), NULL, NULL, 0)) == 0) {
+					return zend_verify_arg_error(E_RECOVERABLE_ERROR, zf, arg_num, "be of the type long", "", zend_zval_type_name(arg), "" TSRMLS_CC);
+				}
+				break;
+
 			case IS_ARRAY:
 				if (!arg) {
 					return zend_verify_arg_error(E_RECOVERABLE_ERROR, zf, arg_num, "be of the type array", "", "none", "" TSRMLS_CC);
diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index 0d2b53a7ca929..2790f2a3faf4d 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -325,14 +325,14 @@ ZEND_END_ARG_INFO()
 ZEND_BEGIN_ARG_INFO_EX(arginfo_mb_substr, 0, 0, 2)
 	ZEND_ARG_INFO(0, str)
 	ZEND_ARG_INFO(0, start)
-	ZEND_ARG_INFO(0, length)
+	ZEND_ARG_TYPE_INFO(0, length, IS_LONG, 1)
 	ZEND_ARG_INFO(0, encoding)
 ZEND_END_ARG_INFO()
 
 ZEND_BEGIN_ARG_INFO_EX(arginfo_mb_strcut, 0, 0, 2)
 	ZEND_ARG_INFO(0, str)
 	ZEND_ARG_INFO(0, start)
-	ZEND_ARG_INFO(0, length)
+	ZEND_ARG_TYPE_INFO(0, length, IS_LONG, 1)
 	ZEND_ARG_INFO(0, encoding)
 ZEND_END_ARG_INFO()
 
@@ -2714,10 +2714,11 @@ PHP_FUNCTION(mb_substr)
 	size_t argc = ZEND_NUM_ARGS();
 	char *str, *encoding;
 	long from, len;
+	zval *zlen = NULL;
 	int mblen, str_len, encoding_len;
 	mbfl_string string, result, *ret;
 
-	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sl|ls", &str, &str_len, &from, &len, &encoding, &encoding_len) == FAILURE) {
+	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sl|z!s", &str, &str_len, &from, &zlen, &encoding, &encoding_len) == FAILURE) {
 		return;
 	}
 
@@ -2736,8 +2737,10 @@ PHP_FUNCTION(mb_substr)
 	string.val = (unsigned char *)str;
 	string.len = str_len;
 
-	if (argc < 3) {
+	if (argc < 3 || !zlen) {
 		len = str_len;
+	} else {
+		len = Z_LVAL_P(zlen);
 	}
 
 	/* measures length */
@@ -2787,6 +2790,7 @@ PHP_FUNCTION(mb_strcut)
 	size_t argc = ZEND_NUM_ARGS();
 	char *encoding;
 	long from, len;
+	zval *zlen = NULL;
 	int encoding_len;
 	mbfl_string string, result, *ret;
 
@@ -2794,7 +2798,7 @@ PHP_FUNCTION(mb_strcut)
 	string.no_language = MBSTRG(language);
 	string.no_encoding = MBSTRG(current_internal_encoding)->no_encoding;
 
-	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sl|ls", (char **)&string.val, (int **)&string.len, &from, &len, &encoding, &encoding_len) == FAILURE) {
+	if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sl|z!s", (char **)&string.val, (int **)&string.len, &from, &zlen, &encoding, &encoding_len) == FAILURE) {
 		return;
 	}
 
@@ -2806,8 +2810,10 @@ PHP_FUNCTION(mb_strcut)
 		}
 	}
 
-	if (argc < 3) {
+	if (argc < 3 || !zlen) {
 		len = string.len;
+	} else {
+		len = Z_LVAL_P(zlen);
 	}
 
 	/* if "from" position is negative, count start position from the end
diff --git a/ext/mbstring/tests/mb_str_functions_opt-parameter.phpt b/ext/mbstring/tests/mb_str_functions_opt-parameter.phpt
index e4a235df308d7..5fb642f9b2ef3 100644
--- a/ext/mbstring/tests/mb_str_functions_opt-parameter.phpt
+++ b/ext/mbstring/tests/mb_str_functions_opt-parameter.phpt
@@ -28,5 +28,3 @@ baz
 baz
 foo
 ==DONE==
---XFAIL--
-mb functions fail to allow null instead of actual value