diff --git a/spec.md b/spec.md
index 8fc02019..46df17b1 100644
--- a/spec.md
+++ b/spec.md
@@ -279,6 +279,7 @@ Program should at least implement these traps:
 - Invalid instruction exception
 - Load address exception
 - Store address exception
+- Unreachable instruction
 
 and executing environment should be able to get information about them,
 like the opcode of invalid instruction or attempted address to load/store.