diff --git a/chapter7/poly/src/Parser.hs b/chapter7/poly/src/Parser.hs
index f24fe6a..107439d 100644
--- a/chapter7/poly/src/Parser.hs
+++ b/chapter7/poly/src/Parser.hs
@@ -17,7 +17,7 @@ import Lexer
 import Syntax
 
 integer :: Parser Integer
-integer = Tok.integer lexer
+integer = Tok.natural lexer
 
 variable :: Parser Expr
 variable = do
diff --git a/chapter7/poly_constraints/src/Parser.hs b/chapter7/poly_constraints/src/Parser.hs
index f24fe6a..107439d 100644
--- a/chapter7/poly_constraints/src/Parser.hs
+++ b/chapter7/poly_constraints/src/Parser.hs
@@ -17,7 +17,7 @@ import Lexer
 import Syntax
 
 integer :: Parser Integer
-integer = Tok.integer lexer
+integer = Tok.natural lexer
 
 variable :: Parser Expr
 variable = do