diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index db91639dc0..fa629324a7 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1168,7 +1168,7 @@ class cpp_function : public function {
         if (!result) {
             std::string msg = "Unable to convert function return value to a "
                               "Python type! The signature was\n\t";
-            msg += it->signature;
+            msg += (it) ? it->signature : std::string("unavailable");
             append_note_if_missing_header_is_suspected(msg);
             // Attach additional error info to the exception if supported
             if (PyErr_Occurred()) {