diff --git a/luprex/core/cpp/traceback.cpp b/luprex/core/cpp/traceback.cpp
index 3f454acd..4842c04b 100644
--- a/luprex/core/cpp/traceback.cpp
+++ b/luprex/core/cpp/traceback.cpp
@@ -1,6 +1,7 @@
 #include "traceback.hpp"
 
 #include <cstring>
+#include <cassert>
 
 #define TRACEBACK_LEVELS1 12
 #define TRACEBACK_LEVELS2 10
@@ -88,7 +89,9 @@ eng::string traceback_pcall(lua_State *L, int narg, int nret) {
         if ((msg == NULL) || (msg[0] == 0)) {
             msg = "unknown error";
         }
-        return msg;
+        eng::string result = msg;
+        assert(result != "attempt to yield from outside a coroutine");
+        return result;
     }
     return "";
 }
diff --git a/luprex/core/cpp/traceback.hpp b/luprex/core/cpp/traceback.hpp
index fb6e6bbb..978b9a5b 100644
--- a/luprex/core/cpp/traceback.hpp
+++ b/luprex/core/cpp/traceback.hpp
@@ -25,7 +25,8 @@ int traceback_coroutine(lua_State *L);
 // Similar to lua_pcall, except that it automatically supplies 
 // traceback_coroutine as a message handler.  It also automatically
 // returns any error message.  Returns empty string if there's
-// no error.
+// no error.  Also checks for a yield inside a pcall, which is 
+// not allowed, and does an assert-fail in that case.
 //
 eng::string traceback_pcall(lua_State *L, int narg, int nret);
 
diff --git a/luprex/core/cpp/world-core.cpp b/luprex/core/cpp/world-core.cpp
index e53d206f..afb3d731 100644
--- a/luprex/core/cpp/world-core.cpp
+++ b/luprex/core/cpp/world-core.cpp
@@ -304,7 +304,6 @@ eng::string World::probe_lua(int64_t actor_id, const eng::string &lua) {
             (*ostream) << std::endl;
         }
     } else {
-        assert(msg != "attempt to yield from outside a coroutine");
         (*ostream) << msg << std::endl;
     }
 
@@ -366,7 +365,6 @@ void World::update_gui(int64_t actor_id, int64_t place_id, Gui *gui) {
     Gui::store_global_pointer(L, nullptr);
     if (!msg.empty()) {
         gui->clear(0);
-        assert(msg != "attempt to yield from outside a coroutine");
         std::cerr << msg << std::endl;
         LS.result();
         return;