diff --git a/dust-lang/src/abstract_tree/built_in_function.rs b/dust-lang/src/abstract_tree/built_in_function.rs
index d69b202..b8fc9e9 100644
--- a/dust-lang/src/abstract_tree/built_in_function.rs
+++ b/dust-lang/src/abstract_tree/built_in_function.rs
@@ -245,7 +245,7 @@ impl FunctionLogic for JsonParse {
         };
 
         Type::Function {
-            type_parameters: None,
+            type_parameters: Some(vec![Identifier::from("T")]),
             value_parameters: Some(vec![(Identifier::from("input"), type_t.clone())]),
             return_type: Some(Box::new(type_t)),
         }
diff --git a/dust-lang/src/abstract_tree/function_call.rs b/dust-lang/src/abstract_tree/function_call.rs
index 5506b16..9fec472 100644
--- a/dust-lang/src/abstract_tree/function_call.rs
+++ b/dust-lang/src/abstract_tree/function_call.rs
@@ -65,19 +65,38 @@ impl AbstractNode for FunctionCall {
             return_type: _,
         } = function_node_type
         {
-            if let (Some(parameters), Some(arguments)) = (type_parameters, &self.type_arguments) {
-                if parameters.len() != arguments.len() {
+            match (type_parameters, &self.type_arguments) {
+                (Some(parameters), Some(arguments)) => {
+                    if parameters.len() != arguments.len() {
+                        return Err(ValidationError::WrongTypeArguments {
+                            arguments: arguments.clone(),
+                            parameters: parameters.clone(),
+                        });
+                    }
+
+                    for (identifier, constructor) in parameters.into_iter().zip(arguments.iter()) {
+                        let r#type = constructor.construct(context)?;
+
+                        context.set_type(
+                            identifier,
+                            r#type,
+                            self.function_expression.position(),
+                        )?;
+                    }
+                }
+                (Some(parameters), None) => {
                     return Err(ValidationError::WrongTypeArguments {
-                        arguments: arguments.clone(),
+                        arguments: Vec::with_capacity(0),
                         parameters: parameters.clone(),
                     });
                 }
-
-                for (identifier, constructor) in parameters.into_iter().zip(arguments.iter()) {
-                    let r#type = constructor.construct(context)?;
-
-                    context.set_type(identifier, r#type, self.function_expression.position())?;
+                (None, Some(arguments)) => {
+                    return Err(ValidationError::WrongTypeArguments {
+                        arguments: arguments.clone(),
+                        parameters: Vec::with_capacity(0),
+                    });
                 }
+                (None, None) => {}
             }
 
             match (value_parameters, &self.value_arguments) {
diff --git a/dust-lang/src/abstract_tree/type.rs b/dust-lang/src/abstract_tree/type.rs
index 00fea89..3855ad6 100644
--- a/dust-lang/src/abstract_tree/type.rs
+++ b/dust-lang/src/abstract_tree/type.rs
@@ -43,8 +43,20 @@ pub enum Type {
 }
 
 impl Type {
+    /// Returns a concrete type, either the type itself or the concrete type of a generic type.
+    pub fn concrete_type(&self) -> &Type {
+        match self {
+            Type::Generic {
+                concrete_type: Some(concrete_type),
+                ..
+            } => concrete_type.concrete_type(),
+            _ => self,
+        }
+    }
+
+    /// Checks that the type is compatible with another type.
     pub fn check(&self, other: &Type) -> Result<(), TypeConflict> {
-        match (self, other) {
+        match (self.concrete_type(), other.concrete_type()) {
             (Type::Any, _)
             | (_, Type::Any)
             | (Type::Boolean, Type::Boolean)
diff --git a/dust-lang/src/lib.rs b/dust-lang/src/lib.rs
index 37d1e3d..4b964fb 100644
--- a/dust-lang/src/lib.rs
+++ b/dust-lang/src/lib.rs
@@ -124,6 +124,10 @@ pub struct InterpreterError {
 }
 
 impl InterpreterError {
+    pub fn new(source_id: Arc<str>, errors: Vec<DustError>) -> Self {
+        InterpreterError { source_id, errors }
+    }
+
     pub fn errors(&self) -> &Vec<DustError> {
         &self.errors
     }
@@ -477,6 +481,8 @@ impl InterpreterError {
 
 #[cfg(test)]
 mod tests {
+    use abstract_tree::{AbstractNode, SourcePosition};
+
     use self::standard_library::std_full_compiled;
 
     use super::*;
@@ -486,6 +492,9 @@ mod tests {
         let context = Context::new();
 
         for abstract_tree in std_full_compiled() {
+            abstract_tree
+                .define_and_validate(&context, true, SourcePosition(0, usize::MAX))
+                .unwrap();
             abstract_tree.run(&context, true).unwrap();
         }
     }
diff --git a/dust-lang/tests/functions.rs b/dust-lang/tests/functions.rs
index 0b357dd..8be61d1 100644
--- a/dust-lang/tests/functions.rs
+++ b/dust-lang/tests/functions.rs
@@ -1,4 +1,7 @@
+use abstract_tree::{Expression, ValueNode, WithPos};
 use dust_lang::*;
+use error::{DustError, ValidationError};
+use identifier::Identifier;
 
 #[test]
 fn function_scope() {
@@ -109,3 +112,54 @@ fn recursion() {
         Ok(Some(Value::integer(13)))
     );
 }
+
+#[test]
+fn value_argument_error() {
+    assert_eq!(
+        interpret(
+            "test",
+            "
+            foobar = fn (a: int, b: int) -> int { a + b }
+            foobar(1)
+            "
+        ),
+        Err(InterpreterError::new(
+            "test".into(),
+            vec![DustError::Validation {
+                error: ValidationError::WrongValueArguments {
+                    parameters: vec![
+                        (Identifier::new("a"), Type::Integer),
+                        (Identifier::new("b"), Type::Integer),
+                    ],
+                    arguments: vec![Expression::Value(
+                        ValueNode::Integer(1).with_position((78, 79)),
+                    )],
+                },
+                position: (71, 80).into()
+            }]
+        ))
+    );
+}
+
+#[test]
+fn type_argument_error() {
+    assert_eq!(
+        interpret(
+            "test",
+            "
+            foobar = fn <T> (a: T) -> T { a }
+            foobar(1)
+            "
+        ),
+        Err(InterpreterError::new(
+            "test".into(),
+            vec![DustError::Validation {
+                error: ValidationError::WrongTypeArguments {
+                    parameters: vec![Identifier::new("T")],
+                    arguments: vec![]
+                },
+                position: (59, 68).into()
+            }]
+        ))
+    );
+}