diff --git a/src/uu/expr/src/syntax_tree.rs b/src/uu/expr/src/syntax_tree.rs
index c55fb0bdc6a..119f325a3d1 100644
--- a/src/uu/expr/src/syntax_tree.rs
+++ b/src/uu/expr/src/syntax_tree.rs
@@ -332,8 +332,12 @@ fn push_token_to_either_stack(
         }
 
         Token::PrefixOp { .. } | Token::ParOpen => {
-            op_stack.push((token_idx, token.clone()));
-            Ok(())
+            if out_stack.is_empty() {
+                op_stack.push((token_idx, token.clone()));
+                Ok(())
+            } else {
+                Err(String::from("syntax error (operation should be prefix)"))
+            }
         }
 
         Token::ParClose => move_till_match_paren(out_stack, op_stack),
diff --git a/tests/by-util/test_expr.rs b/tests/by-util/test_expr.rs
index 28cfcf0ec90..8582082c483 100644
--- a/tests/by-util/test_expr.rs
+++ b/tests/by-util/test_expr.rs
@@ -100,6 +100,11 @@ fn test_parenthesis() {
         .args(&["(", "1", "+", "1", ")", "*", "2"])
         .succeeds()
         .stdout_only("4\n");
+
+    new_ucmd!()
+        .args(&["1", "(", ")"])
+        .fails()
+        .stderr_only("expr: syntax error (operation should be prefix)\n");
 }
 
 #[test]
@@ -221,6 +226,11 @@ fn test_index() {
         .args(&["index", "αbcdef_f", "f"])
         .succeeds()
         .stdout_only("6\n");
+
+    new_ucmd!()
+        .args(&["αbcdef", "index", "α"])
+        .fails()
+        .stderr_only("expr: syntax error (operation should be prefix)\n");
 }
 
 #[test]
@@ -234,6 +244,11 @@ fn test_length() {
         .args(&["length", "abcdef"])
         .succeeds()
         .stdout_only("6\n");
+
+    new_ucmd!()
+        .args(&["abcdef", "length"])
+        .fails()
+        .stderr_only("expr: syntax error (operation should be prefix)\n");
 }
 
 #[test]
@@ -271,6 +286,11 @@ fn test_substr() {
         .args(&["substr", "abc", "1", "1"])
         .succeeds()
         .stdout_only("a\n");
+
+    new_ucmd!()
+        .args(&["abc", "substr", "1", "1"])
+        .fails()
+        .stderr_only("expr: syntax error (operation should be prefix)\n");
 }
 
 #[test]