diff --git a/errors_test.go b/errors_test.go
index c38a382..38e1cdb 100644
--- a/errors_test.go
+++ b/errors_test.go
@@ -20,12 +20,14 @@ func TestErrors(t *testing.T) {
 		{"ExpectedFunctionParameters.q", errors.ExpectedFunctionParameters},
 		{"InvalidInstructionIdentifier.q", &errors.InvalidInstruction{Instruction: "abc"}},
 		{"InvalidInstructionNumber.q", &errors.InvalidInstruction{Instruction: "123"}},
+		{"InvalidExpression.q", errors.InvalidExpression},
 		{"MissingAssignValue.q", errors.MissingAssignValue},
 		{"MissingBlockEnd.q", errors.MissingBlockEnd},
 		{"MissingBlockStart.q", errors.MissingBlockStart},
 		{"MissingGroupEnd.q", errors.MissingGroupEnd},
 		{"MissingGroupStart.q", errors.MissingGroupStart},
 		{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "a"}},
+		{"UnknownIdentifier.q", &errors.UnknownIdentifier{Name: "x"}},
 	}
 
 	for _, test := range tests {
diff --git a/src/build/Function.go b/src/build/Function.go
index 1c2dc0c..671c4e0 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -156,29 +156,10 @@ func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
 	parameters := expr.Children[1:]
 
 	for i, parameter := range parameters {
-		switch parameter.Token.Kind {
-		case token.Identifier:
-			name := parameter.Token.Text()
-			variable, exists := f.Variables[name]
+		err := f.ExpressionToRegister(parameter, f.CPU.Syscall[i])
 
-			if !exists {
-				panic("Unknown identifier " + name)
-			}
-
-			if !variable.IsConst {
-				panic("Not implemented yet")
-			}
-
-			n, _ := strconv.Atoi(variable.Value.Token.Text())
-			f.Assembler.MoveRegisterNumber(f.CPU.Syscall[i], uint64(n))
-
-		case token.Number:
-			value := parameter.Token.Text()
-			n, _ := strconv.Atoi(value)
-			f.Assembler.MoveRegisterNumber(f.CPU.Syscall[i], uint64(n))
-
-		default:
-			panic("Unknown expression")
+		if err != nil {
+			return err
 		}
 	}
 
@@ -191,6 +172,59 @@ func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
 	return nil
 }
 
+// ExpressionToRegister moves the result of an expression into the given register.
+func (f *Function) ExpressionToRegister(root *expression.Expression, register cpu.Register) error {
+	if root.IsLeaf() {
+		return f.TokenToRegister(root.Token, register)
+	}
+
+	return errors.New(errors.NotImplemented, f.File, root.Token.Position)
+}
+
+// TokenToRegister moves a token into a register.
+// It only works with identifiers, numbers and strings.
+func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
+	switch t.Kind {
+	case token.Identifier:
+		name := t.Text()
+		variable, exists := f.Variables[name]
+
+		if !exists {
+			return errors.New(&errors.UnknownIdentifier{Name: name}, f.File, t.Position)
+		}
+
+		if !variable.IsConst {
+			return errors.New(errors.NotImplemented, f.File, t.Position)
+		}
+
+		n, err := strconv.Atoi(variable.Value.Token.Text())
+
+		if err != nil {
+			return err
+		}
+
+		f.Assembler.MoveRegisterNumber(register, uint64(n))
+		return nil
+
+	case token.Number:
+		value := t.Text()
+		n, err := strconv.Atoi(value)
+
+		if err != nil {
+			return err
+		}
+
+		f.Assembler.MoveRegisterNumber(register, uint64(n))
+		return nil
+
+	case token.String:
+		return errors.New(errors.NotImplemented, f.File, t.Position)
+
+	default:
+		return errors.New(errors.InvalidExpression, f.File, t.Position)
+	}
+}
+
 // PrintAsm shows the assembly instructions.
 func (f *Function) PrintAsm() {
 	ansi.Bold.Println(f.Name)
diff --git a/src/errors/CompileErrors.go b/src/errors/CompileErrors.go
index 9f43de2..34515d0 100644
--- a/src/errors/CompileErrors.go
+++ b/src/errors/CompileErrors.go
@@ -2,5 +2,7 @@ package errors
 
 var (
 	InvalidStatement   = &Base{"Invalid statement"}
+	InvalidExpression  = &Base{"Invalid expression"}
 	MissingAssignValue = &Base{"Missing assignment value"}
+	NotImplemented     = &Base{"Not implemented"}
 )
diff --git a/src/errors/UnknownIdentifier.go b/src/errors/UnknownIdentifier.go
new file mode 100644
index 0000000..d6f8dc8
--- /dev/null
+++ b/src/errors/UnknownIdentifier.go
@@ -0,0 +1,18 @@
+package errors
+
+import "fmt"
+
+// UnknownIdentifier represents unknown variables.
+type UnknownIdentifier struct {
+	Name        string
+	CorrectName string
+}
+
+// Error generates the string representation.
+func (err *UnknownIdentifier) Error() string {
+	if err.CorrectName != "" {
+		return fmt.Sprintf("Unknown variable '%s', did you mean '%s'?", err.Name, err.CorrectName)
+	}
+
+	return fmt.Sprintf("Unknown variable '%s'", err.Name)
+}
diff --git a/tests/errors/InvalidExpression.q b/tests/errors/InvalidExpression.q
new file mode 100644
index 0000000..77e99f8
--- /dev/null
+++ b/tests/errors/InvalidExpression.q
@@ -0,0 +1,3 @@
+main() {
+	syscall(+, -, *, /)
+}
\ No newline at end of file
diff --git a/tests/errors/UnknownIdentifier.q b/tests/errors/UnknownIdentifier.q
new file mode 100644
index 0000000..9b13514
--- /dev/null
+++ b/tests/errors/UnknownIdentifier.q
@@ -0,0 +1,3 @@
+main() {
+	syscall(x)
+}
\ No newline at end of file