diff --git a/lib/cpu/instruction.ml b/lib/cpu/instruction.ml
index 7b6f13d..cfa6069 100644
--- a/lib/cpu/instruction.ml
+++ b/lib/cpu/instruction.ml
@@ -1,6 +1,6 @@
 open Uints
 
-type 'a arg =
+type _ arg =
   | Immediate8  : uint8        -> uint8  arg
   | Immediate16 : uint16       -> uint16 arg
   | Direct8     : uint16       -> uint8  arg
diff --git a/lib/cpu/instruction.mli b/lib/cpu/instruction.mli
index 99cc06e..f9dc0ec 100644
--- a/lib/cpu/instruction.mli
+++ b/lib/cpu/instruction.mli
@@ -1,6 +1,6 @@
 open Uints
 
-type 'a arg =
+type _ arg =
   | Immediate8  : uint8        -> uint8  arg
   | Immediate16 : uint16       -> uint16 arg
   | Direct8     : uint16       -> uint8  arg