diff --git a/app/Engine/Cfnpp.php b/app/Engine/Cfnpp.php
index 5979256..88d1fd5 100644
--- a/app/Engine/Cfnpp.php
+++ b/app/Engine/Cfnpp.php
@@ -46,6 +46,11 @@ class Cfnpp implements ICompile
 
 	public function __construct(IOptions $options)
 	{
+		if (!($options instanceof CfnppOptions))
+		{
+			throw new \Exception('Cfnpp compiler requires CfnppOptions');
+		}
+
 		$this->document = new Document();
 		$this->functions = [];
 
diff --git a/app/Engine/CfnppFunctions.php b/app/Engine/CfnppFunctions.php
index 64cf7ef..f9bb6ec 100644
--- a/app/Engine/CfnppFunctions.php
+++ b/app/Engine/CfnppFunctions.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace App\Engine;
 
-use App\Dom\Document;
 use App\Dom\Node;
 use App\Dom\NodeValue;
 use App\Dom\NodeFunction;
@@ -105,6 +104,10 @@ class CfnppFunctions
 	 */
 	public function f_var(Node $node, NodeFunction $function): ?Node
 	{
+		if (!($function instanceof NodeFunctionValue))
+		{
+			throw new \Exception('!var requires scalar argument');
+		}
 		$value = $this->options->getVariable($function->getValue());
 		return new NodeValue(null, $node->hasName() ? $node->getName() : null, $value);
 	}
diff --git a/app/Engine/CfnppOptions.php b/app/Engine/CfnppOptions.php
index 6abe107..78840ad 100644
--- a/app/Engine/CfnppOptions.php
+++ b/app/Engine/CfnppOptions.php
@@ -23,6 +23,12 @@ class CfnppOptions implements IOptions
 		$this->variables = [];
 	}
 
+	/**
+	 * Set all variables.
+	 *
+	 * @param  array<string,mixed> $variables
+	 * @return void
+	 */
 	public function setVariables(array $variables): void
 	{
 		$this->variables = $variables;
@@ -41,8 +47,8 @@ class CfnppOptions implements IOptions
 	/**
 	 * Retrieve the value of a single variable.
 	 *
-	 * @param  string    $name
-	 * @throws Exception if variable does not exist
+	 * @param  string     $name
+	 * @throws \Exception if variable does not exist
 	 * @return mixed
 	 */
 	public function getVariable(string $name)