diff --git a/crates/deno_task_shell/src/grammar.pest b/crates/deno_task_shell/src/grammar.pest
index dc82856..f00ecd5 100644
--- a/crates/deno_task_shell/src/grammar.pest
+++ b/crates/deno_task_shell/src/grammar.pest
@@ -177,10 +177,10 @@ Stdout = ${ "|" ~ !"|" ~ !"&"}
 StdoutStderr = { "|&" }
 
 RESERVED_WORD = _{
-    If | Then | Else | Elif | Fi | Do | Done |
+    (If | Then | Else | Elif | Fi | Done | Do |
     Case | Esac | While | Until | For |
     Lbrace | Rbrace | Bang | In |
-    StdoutStderr | Stdout
+    StdoutStderr | Stdout) ~ &(WHITESPACE | NEWLINE | EOI)
 }
 
 // Main grammar rules
diff --git a/crates/tests/src/lib.rs b/crates/tests/src/lib.rs
index 19f2560..c472fbb 100644
--- a/crates/tests/src/lib.rs
+++ b/crates/tests/src/lib.rs
@@ -1463,6 +1463,22 @@ async fn test_set() {
         .await;
 }
 
+#[tokio::test]
+async fn test_reserved_substring() {
+    // Test that there is no panic (prefix-dev/shell#256)
+    TestBuilder::new()
+        .command(r#"fiqwertymnbvc bla"#)
+        .assert_exit_code(127)
+        .run()
+        .await;
+
+    TestBuilder::new()
+        .command(r#"forplmoknib bla"#)
+        .assert_exit_code(127)
+        .run()
+        .await;
+}
+
 #[cfg(test)]
 fn no_such_file_error_text() -> &'static str {
     if cfg!(windows) {