diff --git a/include/stdex/parser.hpp b/include/stdex/parser.hpp
index 872464cd8..bbe780a36 100644
--- a/include/stdex/parser.hpp
+++ b/include/stdex/parser.hpp
@@ -832,7 +832,7 @@ namespace stdex
 				stdex_assert(text || start >= end);
 				size_t
 					m = m_str.size(),
-					n = std::min<size_t>(end - start, m);
+					n = start < end ? std::min<size_t>(end - start, m) : 0;
 				bool r = ((flags & match_case_insensitive) ?
 					stdex::strnicmp(&text[start], n, m_str.data(), m, this->m_locale) :
 					stdex::strncmp(&text[start], n, m_str.data(), m)) == 0;
@@ -4527,6 +4527,7 @@ namespace stdex
 				m_rparenthesis->invalidate();
 
 				if (m_plus_sign && m_plus_sign->match(text, this->interval.end, end, flags)) {
+					stdex_assert(text || m_plus_sign->interval.start >= m_plus_sign->interval.end);
 					value.append(&text[m_plus_sign->interval.start], &text[m_plus_sign->interval.end]);
 					safe_value_size = value.size();
 					this->interval.end = m_plus_sign->interval.end;