diff --git a/include/stdex/html.hpp b/include/stdex/html.hpp
index e456f866f..236cba03e 100644
--- a/include/stdex/html.hpp
+++ b/include/stdex/html.hpp
@@ -217,6 +217,7 @@ namespace stdex
 			_In_reads_or_z_opt_(num_chars) const char* src, _In_ size_t num_chars)
 		{
 			num_chars = stdex::strnlen(src, num_chars);
+			stdex_assert(src || !num_chars);
 			dst.reserve(dst.size() + num_chars + (num_chars >> 2));
 			for (size_t i = 0; i < num_chars; ++i) {
 				switch (src[i]) {