diff --git a/src/BBox/qHull/global.cpp b/src/BBox/qHull/global.cpp
index 7f7eead..f2e869c 100644
--- a/src/BBox/qHull/global.cpp
+++ b/src/BBox/qHull/global.cpp
@@ -249,7 +249,7 @@ void qh_initflags(char *command) {
 
   if (command != &qh qhull_command[0]) {
     *qh qhull_command = '\0';
-    strncat(qh qhull_command, command, sizeof(qh qhull_command));
+    strncat(qh qhull_command, command, sizeof(qh qhull_command) - strlen(qh qhull_command) - 1);
   }
   while (*s && !isspace(*s)) /* skip program name */
     s++;