diff --git a/crates/oyster_parser/src/parser.rs b/crates/oyster_parser/src/parser.rs
index 34756cd..50e6a5a 100644
--- a/crates/oyster_parser/src/parser.rs
+++ b/crates/oyster_parser/src/parser.rs
@@ -151,31 +151,28 @@ impl Iterator for Parser<'_> {
         use TokenKind::*;
 
         match self.stack.last()? {
-            // XXX: unify Program and CommandSubstitution to avoid duplication
-            NodeKind::Program => match self.lookahead.kind {
+            NodeKind::Program | NodeKind::CommandSubstitution => match self.lookahead.kind {
                 Whitespace => leaf!(Whitespace),
                 Newlines => leaf!(Newlines),
                 Semicolon => leaf!(Semicolon),
                 Comment => leaf!(Comment),
                 PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Pipeline),
                 Pipe => error!(UnexpectedPipe),
-                ClosingParenthesis => error!(UnexpectedClosingParenthesis),
-                Eof => chain!(None, ret!()), // return silently
-            },
-            NodeKind::CommandSubstitution => match self.lookahead.kind {
-                Whitespace => leaf!(Whitespace),
-                Newlines => leaf!(Newlines),
-                Semicolon => leaf!(Semicolon),
-                Comment => leaf!(Comment),
-                PlainText | DoubleQuote | OpeningParenthesis | EscapedChar => call!(Pipeline),
-                ClosingParenthesis => match self.stack.get(self.stack.len() - 2) {
-                    Some(&NodeKind::DQuotedString) => {
-                        chain_buf!(leaf!(ClosingParenthesis, String), ret!())
-                    }
-                    _ => chain_buf!(leaf!(ClosingParenthesis, Command), ret!()),
+                ClosingParenthesis => match self.stack.last().unwrap() {
+                    NodeKind::Program => error!(UnexpectedClosingParenthesis),
+                    NodeKind::CommandSubstitution => match self.stack.get(self.stack.len() - 2) {
+                        Some(&NodeKind::DQuotedString) => {
+                            chain_buf!(leaf!(ClosingParenthesis, String), ret!())
+                        }
+                        _ => chain_buf!(leaf!(ClosingParenthesis, Command), ret!()),
+                    },
+                    _ => unreachable!(),
+                },
+                Eof => match self.stack.last().unwrap() {
+                    NodeKind::Program => chain!(None, ret!()), // return silently
+                    NodeKind::CommandSubstitution => chain_buf!(error!(UnexpectedEof), ret!()),
+                    _ => unreachable!(),
                 },
-                Pipe => error!(UnexpectedPipe),
-                Eof => chain_buf!(error!(UnexpectedEof), ret!()),
             },
             NodeKind::Pipeline => match self.lookahead.kind {
                 Whitespace => leaf!(Whitespace),