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 c435cfd..ba29fdc 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
@@ -41,7 +41,8 @@
     return asList(new Object[][] {
       { "0x1", 1L },
       { "0xA", 10L },
-      { "0xFF", 255L }
+      { "0xFF", 255L },
+      { "0xffffffffffffffff", -1L }
     });
   }
 
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 a142e70..12f6261 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
@@ -14,6 +14,8 @@
 import org.eclipse.xtext.conversion.impl.AbstractLexerBasedConverter;
 import org.eclipse.xtext.nodemodel.INode;
 
+import java.math.BigInteger;
+
 /**
  * Converts hexadecimal numbers to {@code long}s.
  *
@@ -31,13 +33,20 @@
    */
   public Long toValue(String string, INode node) throws ValueConverterException {
     if (isEmpty(string)) throw new ValueConverterException("Couldn't convert empty string to long.", node, null);
+    if (!string.startsWith("0x") && !string.startsWith("0X")) throw parsingError(string, node);
+    String withoutZeroX = string.substring(2, string.length());
     try {
-      return Long.decode(string);
+      BigInteger value = new BigInteger(withoutZeroX, 16);
+      return value.longValue();
     } catch (NumberFormatException e) {
       throw parsingError(string, node, e);
     }
   }
 
+  private ValueConverterException parsingError(String string, INode node) {
+    return parsingError(string, node, null);
+  }
+
   private ValueConverterException parsingError(String string, INode node, Exception cause) {
     return new ValueConverterException("Couldn't convert '" + string + "' to long.", node, cause);
   }