diff --git a/PanoPainter-OSX/main.cpp b/PanoPainter-OSX/main.cpp
index 13609f3..300905e 100644
--- a/PanoPainter-OSX/main.cpp
+++ b/PanoPainter-OSX/main.cpp
@@ -385,7 +385,8 @@ static CVReturn MyDisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTime
 {
     auto keyCode = [theEvent keyCode];
     auto chars = [theEvent characters];
-    std::string s = [chars cStringUsingEncoding:NSASCIIStringEncoding];
+    const char* c_str = [chars cStringUsingEncoding:NSASCIIStringEncoding];
+    std::string s = c_str ? c_str : "";
     std::lock_guard<std::mutex> lock(task_mutex);
     tasklist.emplace_back([keyCode, s] {
         if (App::I.keys[(int)kKey::KeyCtrl])