diff --git a/engine.cc b/engine.cc
index 74b65a3..40845de 100644
--- a/engine.cc
+++ b/engine.cc
@@ -334,6 +334,7 @@ void Engine::regexp() {
 
               char *part = (char *)malloc(substring_length+1);
               part = strncpy(part, (char *)substring_start, substring_length);
+              part[substring_length] = '\0';
 
               subs[i-1] = part;
               free(part);