diff --git a/src/lib/error.hpp b/src/lib/error.hpp
index 0bd3911..4dd80a0 100644
--- a/src/lib/error.hpp
+++ b/src/lib/error.hpp
@@ -91,6 +91,10 @@ namespace ErrorCodes {
     const uint8_t FILE_NOT_OPEN = 9;
     const uint8_t FILE_NOT_FOUND = 10;
 
+    // when performing an operation that would technically be valid but must
+    // never be performed (like deleting an end tag from an NBT compound)
+    const uint8_t NOT_ALLOWED = 11;
+
     const uint8_t UNIMPLEMENTED = 254;
 
     const uint8_t UNKNOWN = 255;