diff --git a/src/plugins/c/items/c_item_string.cpp b/src/plugins/c/items/c_item_string.cpp
index 91eac79..a0fbab5 100644
--- a/src/plugins/c/items/c_item_string.cpp
+++ b/src/plugins/c/items/c_item_string.cpp
@@ -54,6 +54,7 @@ CItemString::GenerateSetterDefinition(std::string message_id) const
     std::stringstream ss;
     // set_xxx_data
     ss << "void " << message_id << "_set_" << id() << "_data(" << message_id << "* message, const void* " << id() << ", int32_t " << id() << "_len) {" << std::endl;
+    ss << "    assert(" << id() << " || " << id() << "_len == 0);" << std::endl;
     ss << "    char* new_" << id() << " = (char*)malloc(" << id() << "_len + 1);" << std::endl;
     ss << "    if (new_" << id() << " == NULL) { return; }" << std::endl;
     ss << "    memcpy(new_" << id() << ", " << id() << ", " << id() << "_len);" << std::endl;