diff --git a/README.md b/README.md
index cc7a4b7..56bbc6f 100644
--- a/README.md
+++ b/README.md
@@ -282,7 +282,7 @@ This crate offers a set of builtin functions.
 |------------|-----------------|----------------|-------------|
 | `min` | >= 1 | Numeric | Returns the minimum of the arguments |
 | `max` | >= 1 | Numeric | Returns the maximum of the arguments |
-| `len` | 1 | String | Returns the character length of a string |
+| `len` | 1 | String/Tuple | Returns the character length of a string, or the amount of elements in a tuple (not recursively) |
 | `str::regex_matches` | 2 | String, String | Returns true if the first argument matches the regex in the second argument |
 | `str::regex_replace` | 3 | String, String, String | Returns the first argument with all matches of the regex in the second argument replaced by the third argument |
 | `str::to_lowercase` | 1 | String | Returns the lower-case version of the string |
diff --git a/src/error/mod.rs b/src/error/mod.rs
index 7baef0d..359dcec 100644
--- a/src/error/mod.rs
+++ b/src/error/mod.rs
@@ -5,10 +5,7 @@
 //! The module also contains some helper functions starting with `expect_` that check for a condition and return `Err(_)` if the condition is not fulfilled.
 //! They are meant as shortcuts to not write the same error checking code everywhere.
 
-use crate::{
-    token::PartialToken,
-    value::{value_type::ValueType, TupleType},
-};
+use crate::{token::PartialToken, value::value_type::ValueType};
 
 use crate::value::Value;
 
@@ -110,7 +107,7 @@ pub enum EvalexprError {
     /// Only use this if there is no other error that describes the expected and provided types in more detail.
     TypeError {
         /// The expected types.
-        expected: TupleType,
+        expected: Vec<ValueType>,
         /// The actual value.
         actual: Value,
     },
@@ -205,7 +202,7 @@ impl EvalexprError {
     }
 
     /// Constructs `Error::TypeError{actual, expected}`.
-    pub fn type_error(actual: Value, expected: TupleType) -> Self {
+    pub fn type_error(actual: Value, expected: Vec<ValueType>) -> Self {
         EvalexprError::TypeError { actual, expected }
     }
 
diff --git a/src/function/builtin.rs b/src/function/builtin.rs
index 4d61d6c..a5f76c5 100644
--- a/src/function/builtin.rs
+++ b/src/function/builtin.rs
@@ -3,7 +3,7 @@ use regex::Regex;
 
 use crate::{
     value::{FloatType, IntType},
-    EvalexprError, Function, Value,
+    EvalexprError, Function, Value, ValueType,
 };
 
 pub fn builtin_function(identifier: &str) -> Option<Function> {
@@ -54,8 +54,16 @@ pub fn builtin_function(identifier: &str) -> Option<Function> {
         }))),
 
         "len" => Some(Function::new(Box::new(|argument| {
-            let subject = argument.as_string()?;
-            Ok(Value::from(subject.len() as i64))
+            if let Ok(subject) = argument.as_string() {
+                Ok(Value::from(subject.len() as i64))
+            } else if let Ok(subject) = argument.as_tuple() {
+                Ok(Value::from(subject.len() as i64))
+            } else {
+                Err(EvalexprError::type_error(
+                    argument.clone(),
+                    vec![ValueType::String, ValueType::Tuple],
+                ))
+            }
         }))),
 
         // string functions
diff --git a/src/lib.rs b/src/lib.rs
index c0c18c2..2335adf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -269,7 +269,7 @@
 //! |------------|-----------------|----------------|-------------|
 //! | `min` | >= 1 | Numeric | Returns the minimum of the arguments |
 //! | `max` | >= 1 | Numeric | Returns the maximum of the arguments |
-//! | `len` | 1 | String | Returns the character length of a string |
+//! | `len` | 1 | String/Tuple | Returns the character length of a string, or the amount of elements in a tuple (not recursively) |
 //! | `str::regex_matches` | 2 | String, String | Returns true if the first argument matches the regex in the second argument |
 //! | `str::regex_replace` | 3 | String, String, String | Returns the first argument with all matches of the regex in the second argument replaced by the third argument |
 //! | `str::to_lowercase` | 1 | String | Returns the lower-case version of the string |
diff --git a/src/value/value_type.rs b/src/value/value_type.rs
index b6837e7..4ea7395 100644
--- a/src/value/value_type.rs
+++ b/src/value/value_type.rs
@@ -1,7 +1,7 @@
 use crate::Value;
 
 /// The type of a `Value`.
-#[derive(Clone, Copy, Eq, PartialEq)]
+#[derive(Clone, Copy, Eq, PartialEq, Debug)]
 pub enum ValueType {
     /// The `Value::String` type.
     String,
diff --git a/tests/integration.rs b/tests/integration.rs
index 353582e..9a240ca 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -277,6 +277,7 @@ fn test_builtin_functions() {
     assert_eq!(eval("min(4.0, 3)"), Ok(Value::Int(3)));
     assert_eq!(eval("max(4.0, 3)"), Ok(Value::Float(4.0)));
     assert_eq!(eval("len(\"foobar\")"), Ok(Value::Int(6)));
+    assert_eq!(eval("len(\"a\", \"b\")"), Ok(Value::Int(2)));
     assert_eq!(
         eval("str::to_lowercase(\"FOOBAR\")"),
         Ok(Value::from("foobar"))
diff --git a/tests/regex.rs b/tests/regex.rs
index c397825..5a9265a 100644
--- a/tests/regex.rs
+++ b/tests/regex.rs
@@ -27,4 +27,4 @@ fn test_regex_functions() {
         eval("str::regex_replace(\"foobar\", \".*?(i+)\", \"b$1\")"),
         Ok(Value::String("foobar".to_owned()))
     );
-}
\ No newline at end of file
+}