diff --git a/lib/imbecile.rb b/lib/imbecile.rb
index 513d025..bd23dcb 100644
--- a/lib/imbecile.rb
+++ b/lib/imbecile.rb
@@ -17,6 +17,15 @@ require_relative "imbecile/version"
 
 module Imbecile
 
+  # Decoding error.
+  TOKEN_DECODE_ERROR = 0xFFFFFFFD
+
+  # Token ID for a "dropped" token.
+  TOKEN_DROP = 0xFFFFFFFE
+
+  # Invalid token ID.
+  TOKEN_NONE = 0xFFFFFFFF
+
   class Error < RuntimeError
   end
 
diff --git a/lib/imbecile/regex/fa.rb b/lib/imbecile/regex/fa.rb
index 9e829e6..7d11983 100644
--- a/lib/imbecile/regex/fa.rb
+++ b/lib/imbecile/regex/fa.rb
@@ -63,19 +63,17 @@ module Imbecile
         states = enumerate
         states.each do |state, id|
           accepts =
-            if state.accepts
-              if state.accepts.name
-                state.accepts.id
-              else
-                0xFFFFFFFE # drop token
-              end
+            if state.accepts.nil?
+              TOKEN_NONE
+            elsif state.accepts.name
+              state.accepts.id
             else
-              0xFFFFFFFF # not an accepting state
+              TOKEN_DROP
             end
           state_table << {
             transition_table_index: transition_table.size,
             n_transitions: state.transitions.size,
-            accepts: state.accepts ? state.accepts.id : 0xFFFFFFFF,
+            accepts: accepts,
           }
           state.transitions.each do |transition|
             transition_table << {