diff --git a/ext/pdo_firebird/firebird_driver.c b/ext/pdo_firebird/firebird_driver.c
index 36a7a2bebc0..a446622c90e 100644
--- a/ext/pdo_firebird/firebird_driver.c
+++ b/ext/pdo_firebird/firebird_driver.c
@@ -662,7 +662,7 @@ free_statement:
 /* called by the PDO SQL parser to add quotes to values that are copied into SQL */
 static zend_string* firebird_handle_quoter(pdo_dbh_t *dbh, const zend_string *unquoted, enum pdo_param_type paramtype)
 {
-	int qcount = 0;
+	size_t qcount = 0;
 	char const *co, *l, *r;
 	char *c;
 	size_t quotedlen;
@@ -676,6 +676,10 @@ static zend_string* firebird_handle_quoter(pdo_dbh_t *dbh, const zend_string *un
 	/* count the number of ' characters */
 	for (co = ZSTR_VAL(unquoted); (co = strchr(co,'\'')); qcount++, co++);
 
+	if (UNEXPECTED(ZSTR_LEN(unquoted) + 2 > ZSTR_MAX_LEN - qcount)) {
+		return NULL;
+	}
+
 	quotedlen = ZSTR_LEN(unquoted) + qcount + 2;
 	quoted_str = zend_string_alloc(quotedlen, 0);
 	c = ZSTR_VAL(quoted_str);