diff --git a/engine/src/functions/concat.rs b/engine/src/functions/concat.rs
index d66ffcfe..4b9ae68f 100644
--- a/engine/src/functions/concat.rs
+++ b/engine/src/functions/concat.rs
@@ -41,7 +41,8 @@ fn concat_bytes<'a>(mut accumulator: Cow<'a, [u8]>, args: FunctionArgs<'_, 'a>)
     accumulator
 }
 
-const EXPECTED_TYPES: [ExpectedType; 2] = [ExpectedType::Array, ExpectedType::Type(Type::Bytes)];
+pub(crate) const EXPECTED_TYPES: [ExpectedType; 2] =
+    [ExpectedType::Array, ExpectedType::Type(Type::Bytes)];
 
 impl FunctionDefinition for ConcatFunction {
     fn check_param(
diff --git a/engine/src/functions/mod.rs b/engine/src/functions/mod.rs
index f3b68dca..482794fa 100644
--- a/engine/src/functions/mod.rs
+++ b/engine/src/functions/mod.rs
@@ -1,6 +1,6 @@
-mod all;
-mod any;
-mod concat;
+pub(crate) mod all;
+pub(crate) mod any;
+pub(crate) mod concat;
 
 use crate::{
     filter::CompiledValueResult,
diff --git a/engine/src/scheme.rs b/engine/src/scheme.rs
index 2924eb5a..ce73c9f1 100644
--- a/engine/src/scheme.rs
+++ b/engine/src/scheme.rs
@@ -628,15 +628,20 @@ macro_rules! Scheme {
 
 #[test]
 fn test_parse_error() {
-    use crate::types::TypeMismatchError;
+    use crate::types::{ExpectedTypeList, TypeMismatchError};
+    use crate::ConcatFunction;
     use indoc::indoc;
 
-    let scheme = &Scheme! {
+    let mut scheme = Scheme! {
         num: Int,
         str: Bytes,
         arr: Array(Bool),
     };
 
+    scheme
+        .add_function("concat", ConcatFunction::new())
+        .unwrap();
+
     {
         let err = scheme.parse("xyz").unwrap_err();
         assert_eq!(
@@ -768,7 +773,7 @@ fn test_parse_error() {
                 r#"
                 Filter parsing error (1:12):
                 arr and arr
-                           ^ expected value of type {Type(Bool)}, but got Array(Bool)
+                           ^ expected value of type Bool, but got Array<Bool>
                 "#
             )
         );
@@ -795,7 +800,7 @@ fn test_parse_error() {
                 r#"
                 Filter parsing error (1:2):
                  arr[*] 
-                 ^^^^^^ expected value of type {Type(Bool)}, but got Array(Bool)
+                 ^^^^^^ expected value of type Bool, but got Array<Bool>
                 "#
             )
         );
@@ -872,6 +877,38 @@ fn test_parse_error() {
             )
         );
     }
+
+    {
+        let err = scheme.parse(indoc!(r"concat(0, 0) == 0")).unwrap_err();
+        assert_eq!(
+            err,
+            ParseError {
+                kind: LexErrorKind::InvalidArgumentType {
+                    index: 0,
+                    mismatch: TypeMismatchError {
+                        expected: ExpectedTypeList::from(
+                            crate::functions::concat::EXPECTED_TYPES.into_iter()
+                        ),
+                        actual: Type::Int,
+                    },
+                },
+                input: "concat(0, 0) == 0",
+                line_number: 0,
+                span_start: 7,
+                span_len: 1,
+            }
+        );
+        assert_eq!(
+            err.to_string(),
+            indoc!(
+                r#"
+                Filter parsing error (1:8):
+                concat(0, 0) == 0
+                       ^ invalid type of argument #0: expected value of type Bytes or Array<_>, but got Int
+                "#
+            )
+        );
+    }
 }
 
 #[test]
diff --git a/engine/src/types.rs b/engine/src/types.rs
index 0223e6e5..27367279 100644
--- a/engine/src/types.rs
+++ b/engine/src/types.rs
@@ -56,6 +56,16 @@ impl From<Type> for ExpectedType {
     }
 }
 
+impl std::fmt::Display for ExpectedType {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            ExpectedType::Array => write!(f, "Array<_>"),
+            ExpectedType::Map => write!(f, "Map<_>"),
+            ExpectedType::Type(ty) => write!(f, "{}", ty),
+        }
+    }
+}
+
 /// A list of expected types.
 #[derive(Default, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct ExpectedTypeList(BTreeSet<ExpectedType>);
@@ -96,7 +106,25 @@ impl<T: Iterator<Item = ExpectedType>> From<T> for ExpectedTypeList {
 
 impl std::fmt::Display for ExpectedTypeList {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        self.0.fmt(f)
+        match self.0.len() {
+            0 => unreachable!(),
+            1 => write!(f, "{}", self.0.first().unwrap()),
+            2 => write!(
+                f,
+                "{} or {}",
+                self.0.first().unwrap(),
+                self.0.last().unwrap()
+            ),
+            _ => {
+                let mut iter = self.0.iter();
+                let first = iter.next().unwrap();
+                write!(f, "{{{}", first)?;
+                for expected_type in iter {
+                    write!(f, ", {}", expected_type)?;
+                }
+                write!(f, "}}")
+            }
+        }
     }
 }
 
@@ -399,8 +427,8 @@ impl std::fmt::Display for Type {
             Self::Bytes => write!(f, "Bytes"),
             Self::Int => write!(f, "Int"),
             Self::Ip => write!(f, "Ip"),
-            Self::Array(ty) => write!(f, "Array({})", Type::from(*ty)),
-            Self::Map(ty) => write!(f, "Map({})", Type::from(*ty)),
+            Self::Array(ty) => write!(f, "Array<{}>", Type::from(*ty)),
+            Self::Map(ty) => write!(f, "Map<{}>", Type::from(*ty)),
         }
     }
 }