diff --git a/README.md b/README.md
index 20fb8d1e..a0fb6836 100644
--- a/README.md
+++ b/README.md
@@ -545,6 +545,9 @@ print(add(2, 3));
 ```
 
 Remember that functions defined in script always take `Dynamic` arguments (i.e. the arguments can be of any type).
+
+Arguments are passed by value, so all functions are _pure_ (i.e. they never modify their arguments).
+
 Furthermore, functions can only be defined at the top level, never inside a block or another function.
 
 ```rust
diff --git a/src/api.rs b/src/api.rs
index 062d28cc..99054a31 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -144,11 +144,6 @@ impl Engine {
             .map_err(|err| EvalAltResult::ErrorParsing(err))
             .and_then(|AST(ref os, ref fns)| {
                 for f in fns {
-                    // FIX - Why are functions limited to 6 parameters?
-                    if f.params.len() > 6 {
-                        return Ok(());
-                    }
-
                     self.script_fns.insert(
                         FnSpec {
                             ident: f.name.clone(),
diff --git a/src/engine.rs b/src/engine.rs
index 8388f2e5..dfdf0309 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -21,7 +21,7 @@ const KEYWORD_TYPE_OF: &'static str = "type_of";
 pub enum EvalAltResult {
     ErrorParsing(ParseError),
     ErrorFunctionNotFound(String, Position),
-    ErrorFunctionArgsMismatch(String, usize, Position),
+    ErrorFunctionArgsMismatch(String, usize, usize, Position),
     ErrorBooleanArgMismatch(String, Position),
     ErrorArrayBounds(usize, i64, Position),
     ErrorStringBounds(usize, i64, Position),
@@ -45,7 +45,7 @@ impl Error for EvalAltResult {
         match self {
             Self::ErrorParsing(p) => p.description(),
             Self::ErrorFunctionNotFound(_, _) => "Function not found",
-            Self::ErrorFunctionArgsMismatch(_, _, _) => {
+            Self::ErrorFunctionArgsMismatch(_, _, _, _) => {
                 "Function call with wrong number of arguments"
             }
             Self::ErrorBooleanArgMismatch(_, _) => "Boolean operator expects boolean operands",
@@ -105,9 +105,11 @@ impl std::fmt::Display for EvalAltResult {
                 write!(f, "{} '{}': {}", desc, filename, err)
             }
             Self::ErrorParsing(p) => write!(f, "Syntax error: {}", p),
-            Self::ErrorFunctionArgsMismatch(fun, n, pos) => {
-                write!(f, "Function '{}' expects {} argument(s) ({})", fun, n, pos)
-            }
+            Self::ErrorFunctionArgsMismatch(fun, need, n, pos) => write!(
+                f,
+                "Function '{}' expects {} argument(s) but {} found ({})",
+                fun, need, n, pos
+            ),
             Self::ErrorBooleanArgMismatch(op, pos) => {
                 write!(f, "{} operator expects boolean operands ({})", op, pos)
             }
@@ -244,6 +246,15 @@ impl Engine {
                     .into_dynamic())
                 }
                 FnIntExt::Int(ref f) => {
+                    if f.params.len() != args.len() {
+                        return Err(EvalAltResult::ErrorFunctionArgsMismatch(
+                            spec.ident,
+                            f.params.len(),
+                            args.len(),
+                            pos,
+                        ));
+                    }
+
                     let mut scope = Scope::new();
 
                     scope.extend(
diff --git a/src/fn_register.rs b/src/fn_register.rs
index ee08dfd1..4e28b05f 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -39,7 +39,7 @@ macro_rules! def_register {
                     const NUM_ARGS: usize = count_args!($($par)*);
 
                     if args.len() != NUM_ARGS {
-                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, pos))
+                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, args.len(), pos))
                     } else {
                         #[allow(unused_variables, unused_mut)]
                         let mut drain = args.drain(..);
@@ -72,7 +72,7 @@ macro_rules! def_register {
                     const NUM_ARGS: usize = count_args!($($par)*);
 
                     if args.len() != NUM_ARGS {
-                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, pos))
+                        Err(EvalAltResult::ErrorFunctionArgsMismatch(fn_name.clone(), NUM_ARGS, args.len(), pos))
                     } else {
                         #[allow(unused_variables, unused_mut)]
                         let mut drain = args.drain(..);