diff --git a/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_Test.java b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_Test.java
index 992040f..50b4256 100644
--- a/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_Test.java
+++ b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_Test.java
@@ -10,9 +10,10 @@
 
 import static java.util.Arrays.asList;
 import static org.hamcrest.core.IsEqual.equalTo;
+import static org.junit.Assert.assertThat;
 import static org.mockito.Mockito.mock;
 
-import com.google.eclipse.protobuf.junit.core.XtextRule;
+import java.util.Collection;
 
 import org.eclipse.xtext.nodemodel.INode;
 import org.junit.*;
@@ -20,11 +21,11 @@
 import org.junit.runners.*;
 import org.junit.runners.Parameterized.Parameters;
 
-import java.util.*;
+import com.google.eclipse.protobuf.junit.core.XtextRule;
 
 /**
  * Tests for <code>{@link HEXValueConverter#toValue(String, INode)}</code>.
- * 
+ *
  * @author alruiz@google.com (Alex Ruiz)
  */
 @RunWith(Parameterized.class)
@@ -34,7 +35,7 @@
 
   private final String hexadecimal;
   private final Integer expected;
-  
+
   @Parameters
   public static Collection<Object[]> parameters() {
     return asList(new Object[][] {
@@ -43,22 +44,22 @@
       { "0xFF", 255 }
     });
   }
-  
+
   public HEXValueConverter_toValue_Test(String hexadecimal, Integer expected) {
     this.hexadecimal = hexadecimal;
     this.expected = expected;
   }
-  
+
   private HEXValueConverter converter;
   private INode node;
-  
+
   @Before public void setUp() {
     node = mock(INode.class);
     converter = xtext.injector().getInstance(HEXValueConverter.class);
   }
-  
+
   @Test public void should_parse_hexadecimal_number() {
     Integer value = converter.toValue(hexadecimal, node);
-    Assert.assertThat(value, equalTo(expected));
+    assertThat(value, equalTo(expected));
   }
 }
diff --git a/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_withInvalidInput_Test.java b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_withInvalidInput_Test.java
new file mode 100644
index 0000000..9a01e03
--- /dev/null
+++ b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/conversion/HEXValueConverter_toValue_withInvalidInput_Test.java
@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2011 Google Inc.
+ *
+ * All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse
+ * Public License v1.0 which accompanies this distribution, and is available at
+ *
+ * http://www.eclipse.org/legal/epl-v10.html
+ */
+package com.google.eclipse.protobuf.conversion;
+
+import static org.hamcrest.core.IsEqual.equalTo;
+import static org.hamcrest.core.IsInstanceOf.instanceOf;
+import static org.junit.Assert.*;
+import static org.junit.rules.ExpectedException.none;
+import static org.mockito.Mockito.mock;
+
+import org.eclipse.xtext.conversion.ValueConverterException;
+import org.eclipse.xtext.nodemodel.INode;
+import org.junit.*;
+import org.junit.rules.ExpectedException;
+
+import com.google.eclipse.protobuf.junit.core.XtextRule;
+
+/**
+ * Tests for <code>{@link HEXValueConverter#toValue(String, INode)}</code>.
+ *
+ * @author alruiz@google.com (Alex Ruiz)
+ */
+public class HEXValueConverter_toValue_withInvalidInput_Test {
+
+  @Rule public XtextRule xtext = new XtextRule();
+  @Rule public ExpectedException thrown = none();
+
+  private HEXValueConverter converter;
+  private INode node;
+
+  @Before public void setUp() {
+    node = mock(INode.class);
+    converter = xtext.injector().getInstance(HEXValueConverter.class);
+  }
+
+  @Test public void should_throw_error_if_input_is_null() {
+    thrown.expect(ValueConverterException.class);
+    thrown.expectMessage("Couldn't convert empty string to int.");
+    converter.toValue(null, node);
+  }
+
+  @Test public void should_throw_error_if_input_is_empty() {
+    thrown.expect(ValueConverterException.class);
+    thrown.expectMessage("Couldn't convert empty string to int.");
+    converter.toValue("", node);
+  }
+
+  @Test public void should_throw_error_if_input_has_less_than_three_characters() {
+    thrown.expect(ValueConverterException.class);
+    thrown.expectMessage("Couldn't convert '0x' to int.");
+    converter.toValue("0x", node);
+  }
+
+  @Test public void should_throw_error_if_input_does_not_start_with_0x() {
+    thrown.expect(ValueConverterException.class);
+    thrown.expectMessage("Couldn't convert '65' to int.");
+    converter.toValue("65", node);
+  }
+
+  @Test public void should_throw_error_if_conversion_throws_NumberFormatException() {
+    try {
+      converter.toValue("0xZ", node);
+      fail("Expecting a " + ValueConverterException.class.getName());
+    } catch (ValueConverterException e) {
+      assertThat(e.getMessage(), equalTo("Couldn't convert '0xZ' to int."));
+      assertThat(e.getCause(), instanceOf(NumberFormatException.class));
+    }
+  }
+}
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
index 92c2cbd..d731b46 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
@@ -15,18 +15,31 @@
 import org.eclipse.xtext.nodemodel.INode;
 
 /**
+ * Converts hexadecimal numbers to {@code int}s.
+ *
  * @author alruiz@google.com (Alex Ruiz)
  */
 public class HEXValueConverter extends AbstractLexerBasedConverter<Integer> {
 
+  /**
+   * Creates am {@code int} from the given input, if the given input represents an hexadecimal number.
+   * @param string the given input.
+   * @param node the parsed node including hidden parts.
+   * @return the new integer.
+   * @throws ValueConverterException if the given input is {@code null}, empty or does not represent an hexadecimal
+   * number.
+   */
   public Integer toValue(String string, INode node) throws ValueConverterException {
     if (isEmpty(string)) throw new ValueConverterException("Couldn't convert empty string to int.", node, null);
     int length = string.length();
     if (length < 3) throw parsingError(string, node);
     if (!string.substring(0, 2).equalsIgnoreCase("0x")) throw parsingError(string, node);
     String val = string.substring(2, length);
-    int parsed = Integer.parseInt(val, 16);
-    return parsed;
+    try {
+      return Integer.parseInt(val, 16);
+    } catch (NumberFormatException e) {
+      throw parsingError(string, node, e);
+    }
   }
 
   private ValueConverterException parsingError(String string, INode node) {