diff --git a/lib/lrama/lexer/token.rb b/lib/lrama/lexer/token.rb
index 6221abe0..a6180746 100644
--- a/lib/lrama/lexer/token.rb
+++ b/lib/lrama/lexer/token.rb
@@ -28,8 +28,13 @@ def numberize_references(lhs, rhs)
               if lhs.referred_by?(ref_name)
                 '$'
               else
-                # TODO: Better error message for wrong reference
-                rhs.find_index {|token| token.referred_by?(ref_name) } + 1
+                index = rhs.find_index {|token| token.referred_by?(ref_name) }
+
+                if index
+                  index + 1
+                else
+                  raise "'#{ref_name}' is invalid name."
+                end
               end
             [ref[0], value, ref[2], ref[3], ref[4]]
           else
diff --git a/spec/lrama/parser_spec.rb b/spec/lrama/parser_spec.rb
index 0c1dd71c..32fe0fe8 100644
--- a/spec/lrama/parser_spec.rb
+++ b/spec/lrama/parser_spec.rb
@@ -1277,6 +1277,43 @@ class : keyword_class tSTRING keyword_end { code 1 }
             ])
           end
         end
+
+        context "includes invalid named references" do
+          it "raise an error" do
+            y = <<~INPUT
+%{
+// Prologue
+%}
+
+%union {
+  int i;
+}
+
+%token NUM
+%type <val> expr
+
+%%
+
+input       : /* empty */
+            | input line
+;
+
+line        : '\\n'
+            | expr '\\n'
+                { printf("\\t%.10g\\n", $expr); }
+;
+
+expr[result]: NUM
+            | expr[left] expr[right] '+'
+                { $results = $left + $right; }
+            | expr expr '-'
+                { $$ = $1 - $2; }
+;
+            INPUT
+
+            expect { Lrama::Parser.new(y).parse }.to raise_error("'results' is invalid name.")
+          end
+        end
       end
     end
   end