diff --git a/src/API.cpp b/src/API.cpp
index fa10f321e..566692710 100644
--- a/src/API.cpp
+++ b/src/API.cpp
@@ -491,10 +491,11 @@ bool API::callProgramHook (
     throw std::string ("Error calling '") + function + "' - " + lua_tostring (L, -1);
 
   // Call successful - get return values.
-  if (!lua_isnumber (L, -2)) throw std::string ("Error: '") + function + "' did not return a success indicator";
+  if (!lua_isnumber (L, -2))
+    throw std::string ("Error: '") + function + "' did not return a success indicator";
 
-// TODO This doesn't seem to know that 'nil' was returned instead of a string.
-//  if (!lua_isstring (L, -1)) throw std::string ("Error: '") + function + "' did not return a message";
+  if (!lua_isstring (L, -1) && !lua_isnil (L, -1))
+    throw std::string ("Error: '") + function + "' did not return a message or nil";
 
   int rc              = lua_tointeger (L, -2);
   const char* message = lua_tostring  (L, -1);