diff --git a/lib/oga/css/parser.y b/lib/oga/css/parser.y
index c986e61..ba393ce 100644
--- a/lib/oga/css/parser.y
+++ b/lib/oga/css/parser.y
@@ -272,7 +272,7 @@ rule
 
   nth
     : T_NTH                 { s(:nth, s(:int, 1)) }
-    | T_MINUS T_NTH         { s(:nth) }
+    | T_MINUS T_NTH         { s(:nth, s(:int, 1)) }
     | integer T_NTH         { s(:nth, val[0]) }
     | integer T_NTH integer { s(:nth, val[0], val[2]) }
     ;
@@ -384,7 +384,7 @@ end
           s(:eq, s(:mod, s(:sub, before_count, offset), s(:int, 2)), s(:int, 0))
         )
       else
-        node = s(:mod, before_count, step)
+        node = s(:eq, s(:mod, before_count, step), s(:int, 0))
       end
     end
 
diff --git a/spec/oga/css/parser/pseudo_classes_spec.rb b/spec/oga/css/parser/pseudo_classes_spec.rb
index fad1a4c..cb698bb 100644
--- a/spec/oga/css/parser/pseudo_classes_spec.rb
+++ b/spec/oga/css/parser/pseudo_classes_spec.rb
@@ -34,7 +34,7 @@ describe Oga::CSS::Parser do
 
     example 'parse the x:nth-child(even) pseudo class' do
       parse_css('x:nth-child(even)').should == parse_xpath(
-        'descendant-or-self::x[(count(preceding-sibling::*) + 1) mod 2]'
+        'descendant-or-self::x[((count(preceding-sibling::*) + 1) mod 2) = 0]'
       )
     end
 
@@ -46,20 +46,14 @@ describe Oga::CSS::Parser do
     end
 
     example 'parse the x:nth-child(n) pseudo class' do
-      parse_css('x:nth-child(n)').should == s(
-        :pseudo,
-        s(:test, nil, 'x'),
-        'nth-child',
-        s(:nth)
+      parse_css('x:nth-child(n)').should == parse_xpath(
+        'descendant-or-self::x[((count(preceding-sibling::*) + 1) mod 1) = 0]'
       )
     end
 
     example 'parse the x:nth-child(-n) pseudo class' do
-      parse_css('x:nth-child(-n)').should == s(
-        :pseudo,
-        s(:test, nil, 'x'),
-        'nth-child',
-        s(:nth)
+      parse_css('x:nth-child(-n)').should == parse_xpath(
+        'descendant-or-self::x[((count(preceding-sibling::*) + 1) mod 1) = 0]'
       )
     end