diff --git a/app/Cfnpp/Compiler.php b/app/Cfnpp/Compiler.php
index 558b7f9..051c593 100644
--- a/app/Cfnpp/Compiler.php
+++ b/app/Cfnpp/Compiler.php
@@ -216,7 +216,7 @@ class Compiler implements \App\Engine\ICompile
 			foreach ($parameters_node as $parameter_node)
 			{
 				$nodes[$parameter_node->getName()] = $parameter_node;
-				$graph->add($parameter_node->getName(), $this->pass_1_getDependencies($parameter_node));
+				$graph->add($parameter_node->getName(), $this->pass_1_getDependencies($parameter_node, $options));
 			}
 		}
 
@@ -229,7 +229,7 @@ class Compiler implements \App\Engine\ICompile
 					throw new \Exception('Variables and parameters cannot share the same name.');
 				}
 				$nodes[$variable_node->getName()] = $variable_node;
-				$graph->add($variable_node->getName(), $this->pass_1_getDependencies($variable_node));
+				$graph->add($variable_node->getName(), $this->pass_1_getDependencies($variable_node, $options));
 			}
 		}
 
@@ -268,7 +268,7 @@ class Compiler implements \App\Engine\ICompile
 	 * @param  Node     $node
 	 * @return string[]
 	 */
-	protected function pass_1_getDependencies(Node $node): array
+	protected function pass_1_getDependencies(Node $node, IOptions $options): array
 	{
 		$stack = [$node];
 
@@ -296,7 +296,7 @@ class Compiler implements \App\Engine\ICompile
 			elseif ($node instanceof NodeFunctionValue &&
 				$node->getName() == 'expr')
 			{
-				$expression = new \App\Cfnpp\Expression\Expression($node->getValue());
+				$expression = new \App\Cfnpp\Expression\Expression($node->getValue(), $options);
 				$variables = array_merge($variables, $expression->getReferencedVariables());
 			}
 		}
diff --git a/app/Cfnpp/Expression/Expression.php b/app/Cfnpp/Expression/Expression.php
index 4f2a7e2..81ba21d 100644
--- a/app/Cfnpp/Expression/Expression.php
+++ b/app/Cfnpp/Expression/Expression.php
@@ -48,12 +48,15 @@ class Expression
 	 * @param string               $expression
 	 * @param \App\Engine\IOptions $options
 	 */
-	public function __construct(string $expression, \App\Engine\IOptions $options)
+	public function __construct(string $expression)
 	{
 		$tokens = static::tokenize($expression);
-		$nodes = static::parse($tokens);
-		$this->nodes = static::solve($nodes, $options->getVariables(), array_keys($options->getParameters()));
-		$this->options = $options;
+		$this->nodes = static::parse($tokens);
+	}
+
+	public function solve(\App\Engine\IOptions $options)
+	{
+		$this->nodes = static::_solve($this->nodes, $options->getVariables(), array_keys($options->getParameters()));
 	}
 
 	/**
@@ -128,6 +131,27 @@ class Expression
 		return static::cloudformation($this->nodes);
 	}
 
+	/**
+	 * Examine the expression to determine which variables are referenced so we
+	 * can figure out the dependencies between them
+	 *
+	 * @return string[] names of variables referenced
+	 */
+	public function getReferencedVariables(): array
+	{
+		$variables = [];
+		foreach ($this->nodes as $node)
+		{
+			$node->walk(function($node) use (&$variables) {
+				if ($node->getValue() instanceof Token\Variable)
+				{
+					$variables[] = $node->getValue()->getName();
+				}
+			});
+		}
+		return array_values(array_unique($variables));
+	}
+
 	/**
 	 * Convert an expression string into a series of tokens.
 	 *
@@ -203,7 +227,7 @@ class Expression
 	 * @param  string[]            $parameters parameter names
 	 * @return GraphNode[]
 	 */
-	protected static function solve(array $nodes, array $variables = [], array $parameters = []): array
+	protected static function _solve(array $nodes, array $variables = [], array $parameters = []): array
 	{
 		$root = new GraphNode();
 		foreach ($nodes as $node)
diff --git a/app/Cfnpp/Functions.php b/app/Cfnpp/Functions.php
index a4469e5..e5eb1a1 100644
--- a/app/Cfnpp/Functions.php
+++ b/app/Cfnpp/Functions.php
@@ -166,7 +166,8 @@ class Functions
 		$if_true = $nodes[1];
 		$if_false = sizeof($nodes) == 3 ? $nodes[2] : null;
 
-		$expression = new \App\Cfnpp\Expression\Expression($condition->getValue(), $this->options);
+		$expression = new \App\Cfnpp\Expression\Expression($condition->getValue());
+		$expression->solve($this->options);
 
 		// We need a single resulting node as a final value either as a scalar or
 		// if we want to convert to cloudformation.
@@ -222,7 +223,8 @@ class Functions
 			throw new \Exception('!expr requires scalar argument');
 		}
 
-		$expression = new \App\Cfnpp\Expression\Expression($function->getValue(), $this->options);
+		$expression = new \App\Cfnpp\Expression\Expression($function->getValue());
+		$expression->solve($this->options);
 
 		if ($expression->isComplete())
 		{