diff --git a/kalk/src/inverter.rs b/kalk/src/inverter.rs
index 11d0b2d..b474c34 100644
--- a/kalk/src/inverter.rs
+++ b/kalk/src/inverter.rs
@@ -1,9 +1,10 @@
 use crate::ast::{Expr, Stmt};
 use crate::lexer::TokenKind;
+use crate::parser::CalcError;
 use crate::symbol_table::SymbolTable;
 
 impl Expr {
-    pub fn invert(&self, symbol_table: &mut SymbolTable) -> Self {
+    pub fn invert(&self, symbol_table: &mut SymbolTable) -> Result<Self, CalcError> {
         match self {
             Expr::Binary(left, op, right) => invert_binary(symbol_table, &left, op, &right),
             Expr::Unary(op, expr) => invert_unary(op, &expr),
@@ -23,7 +24,7 @@ fn invert_binary(
     left: &Expr,
     op: &TokenKind,
     right: &Expr,
-) -> Expr {
+) -> Result<Expr, CalcError> {
     let op_inv = match op {
         TokenKind::Plus => TokenKind::Minus,
         TokenKind::Minus => TokenKind::Plus,
@@ -32,38 +33,50 @@ fn invert_binary(
         _ => unreachable!(),
     };
 
-    Expr::Binary(
-        Box::new(right.invert(symbol_table)),
+    Ok(Expr::Binary(
+        Box::new(right.invert(symbol_table)?),
         op_inv,
-        Box::new(left.invert(symbol_table)),
-    )
+        Box::new(left.invert(symbol_table)?),
+    ))
 }
 
-fn invert_unary(op: &TokenKind, expr: &Expr) -> Expr {
-    match op {
+fn invert_unary(op: &TokenKind, expr: &Expr) -> Result<Expr, CalcError> {
+    Ok(match op {
         TokenKind::Minus => expr.clone(),
         TokenKind::Exclamation => unimplemented!(),
         _ => unreachable!(),
-    }
+    })
 }
 
 // Not necessary yet
-fn invert_unit(_identifier: &str, _expr: &Expr) -> Expr {
+fn invert_unit(_identifier: &str, _expr: &Expr) -> Result<Expr, CalcError> {
     unimplemented!()
 }
 
-fn invert_group(expr: &Expr) -> Expr {
+fn invert_group(expr: &Expr) -> Result<Expr, CalcError> {
     invert_value(expr)
 }
 
-fn invert_fn_call(symbol_table: &mut SymbolTable, identifier: &str, arguments: &Vec<Expr>) -> Expr {
+fn invert_fn_call(
+    symbol_table: &mut SymbolTable,
+    identifier: &str,
+    arguments: &Vec<Expr>,
+) -> Result<Expr, CalcError> {
     let (parameters, body) =
         if let Some(Stmt::FnDecl(_, parameters, body)) = symbol_table.get_fn(identifier).cloned() {
             (parameters, body)
         } else {
-            panic!(); // TODO: Error checking
+            return Err(CalcError::UndefinedFn(identifier.into()));
         };
 
+    if parameters.len() != arguments.len() {
+        return Err(CalcError::IncorrectAmountOfArguments(
+            parameters.len(),
+            identifier.into(),
+            arguments.len(),
+        ));
+    }
+
     let mut parameters_iter = parameters.iter();
     for argument in arguments {
         symbol_table.insert(Stmt::VarDecl(
@@ -75,6 +88,6 @@ fn invert_fn_call(symbol_table: &mut SymbolTable, identifier: &str, arguments: &
     body.invert(symbol_table)
 }
 
-fn invert_value(expr: &Expr) -> Expr {
-    Expr::Unary(TokenKind::Minus, Box::new(expr.clone()))
+fn invert_value(expr: &Expr) -> Result<Expr, CalcError> {
+    Ok(Expr::Unary(TokenKind::Minus, Box::new(expr.clone())))
 }
diff --git a/kalk/src/parser.rs b/kalk/src/parser.rs
index a998799..be474be 100644
--- a/kalk/src/parser.rs
+++ b/kalk/src/parser.rs
@@ -187,7 +187,7 @@ fn parse_unit_decl_stmt(context: &mut Context) -> Result<Stmt, CalcError> {
     let stmt_inv = Stmt::UnitDecl(
         base_unit.clone(),
         identifier.value.clone(),
-        Box::new(def.invert(&mut context.symbol_table)),
+        Box::new(def.invert(&mut context.symbol_table)?),
     );
     let stmt = Stmt::UnitDecl(identifier.value, base_unit, Box::new(def));