diff --git a/src/Nirum/Parser.hs b/src/Nirum/Parser.hs
index ad116c9..e458095 100644
--- a/src/Nirum/Parser.hs
+++ b/src/Nirum/Parser.hs
@@ -412,7 +412,9 @@ tag = do
     spaces
     paren <- optional $ char '('
     fields' <- case paren of
-        Just _ -> do { f <- fieldSet <?> "union tag fields"
+        Just _ -> do { spaces
+                     ; f <- fieldSet <?> "union tag fields"
+                     ; spaces
                      ; char ')'
                      ; return f
                      }
diff --git a/test/Nirum/ParserSpec.hs b/test/Nirum/ParserSpec.hs
index 0259c7e..68f29bb 100644
--- a/test/Nirum/ParserSpec.hs
+++ b/test/Nirum/ParserSpec.hs
@@ -606,6 +606,17 @@ spec = do
                                     \point lower-right,)\n\
                    \    | none\n\
                    \    ;" `shouldBeRight` a
+            parse' "union shape\n\
+                   \    = circle (\n\
+                   \        point origin,\n\
+                   \        offset radius,\n\
+                   \      )\n\
+                   \    | rectangle (\n\
+                   \        point upper-left,\n\
+                   \        point lower-right,\n\
+                   \      )\n\
+                   \    | none\n\
+                   \    ;" `shouldBeRight` a
             parse' "union shape\n\
                    \    # shape type\n\
                    \    = circle (point origin, \