diff --git a/src/main/java/com/annimon/ownlang/lib/StringValue.java b/src/main/java/com/annimon/ownlang/lib/StringValue.java
index 91ca490..d58f59f 100644
--- a/src/main/java/com/annimon/ownlang/lib/StringValue.java
+++ b/src/main/java/com/annimon/ownlang/lib/StringValue.java
@@ -87,20 +87,12 @@ public final class StringValue implements Value {
     
     @Override
     public int asInt() {
-        try {
-            return Integer.parseInt(value);
-        } catch (NumberFormatException e) {
-            return 0;
-        }
+        return Integer.parseInt(value);
     }
     
     @Override
     public double asNumber() {
-        try {
-            return Double.parseDouble(value);
-        } catch (NumberFormatException e) {
-            return 0;
-        }
+        return Double.parseDouble(value);
     }
 
     @Override
diff --git a/src/test/java/com/annimon/ownlang/parser/ParserTest.java b/src/test/java/com/annimon/ownlang/parser/ParserTest.java
index 598a798..9a473d5 100644
--- a/src/test/java/com/annimon/ownlang/parser/ParserTest.java
+++ b/src/test/java/com/annimon/ownlang/parser/ParserTest.java
@@ -35,8 +35,11 @@ public class ParserTest {
         BlockStatement program = assertExpression(input, expected);
         program.execute();
         final Value actual = Variables.get("a");
-        assertEquals(expectedValue.asNumber(), actual.asNumber(), 0.001);
-        assertEquals(expectedValue.asString(), actual.asString());
+        try {
+            assertEquals(expectedValue.asNumber(), actual.asNumber(), 0.001);
+        } catch (NumberFormatException nfe) {
+            assertEquals(expectedValue.asString(), actual.asString());
+        }
     }
     
     private static BlockStatement assertExpression(String input, Expression expected) {
diff --git a/src/test/java/com/annimon/ownlang/parser/ast/VariableExpressionTest.java b/src/test/java/com/annimon/ownlang/parser/ast/VariableExpressionTest.java
index 1478368..36ccbde 100644
--- a/src/test/java/com/annimon/ownlang/parser/ast/VariableExpressionTest.java
+++ b/src/test/java/com/annimon/ownlang/parser/ast/VariableExpressionTest.java
@@ -1,6 +1,7 @@
 package com.annimon.ownlang.parser.ast;
 
 import static com.annimon.ownlang.parser.ast.ASTHelper.*;
+import com.annimon.ownlang.exceptions.VariableDoesNotExistsException;
 import org.junit.Test;
 
 /**
@@ -26,7 +27,7 @@ public class VariableExpressionTest {
         assertValue(number(8), var("a").eval());
     }
     
-    @Test(expected = RuntimeException.class)
+    @Test(expected = VariableDoesNotExistsException.class)
     public void testUnknownVariable() {
         var("a").eval();
     }