2 files changed, 19 insertions, 4 deletions
diff --git a/src/main/java/org/zwobble/couscous/interpreter/Environment.java b/src/main/java/org/zwobble/couscous/interpreter/Environment.java
index d748654..a24bf1c 100644
--- a/src/main/java/org/zwobble/couscous/interpreter/Environment.java
+++ b/src/main/java/org/zwobble/couscous/interpreter/Environment.java
@@ -9,6 +9,8 @@ import org.zwobble.couscous.ast.VariableNode;
 import org.zwobble.couscous.values.ConcreteType;
 import org.zwobble.couscous.values.InterpreterValue;
 
+import lombok.val;
+
 public class Environment {
     private final Map<Integer, Optional<InterpreterValue>> stackFrame;
     private final Project project;
@@ -19,12 +21,13 @@ public class Environment {
     }
 
     public InterpreterValue get(int variableId) {
-        checkVariable(variableId);
-        return stackFrame.get(variableId).get();
+        checkVariableIsInScope(variableId);
+        val value = stackFrame.get(variableId);
+        return value.orElseThrow(() -> new UnboundVariable(variableId));
     }
 
     public void put(int variableId, InterpreterValue value) {
-        checkVariable(variableId);
+        checkVariableIsInScope(variableId);
         stackFrame.put(variableId, Optional.of(value));
     }
 
@@ -41,7 +44,7 @@ public class Environment {
         return new Environment(project, stackFrame);
     }
 
-    private void checkVariable(int variableId) {
+    private void checkVariableIsInScope(int variableId) {
         if (!stackFrame.containsKey(variableId)) {
             throw new VariableNotInScope(variableId);
         }
diff --git a/src/main/java/org/zwobble/couscous/interpreter/UnboundVariable.java b/src/main/java/org/zwobble/couscous/interpreter/UnboundVariable.java
new file mode 100644
index 0000000..c4aebfb
--- /dev/null
+++ b/src/main/java/org/zwobble/couscous/interpreter/UnboundVariable.java
@@ -0,0 +1,12 @@
+package org.zwobble.couscous.interpreter;
+
+import lombok.EqualsAndHashCode;
+import lombok.Value;
+
+@Value
+@EqualsAndHashCode(callSuper=false)
+public class UnboundVariable extends InterpreterException {
+    private static final long serialVersionUID = 1L;
+    
+    int variableId;
+}