diff --git a/ethereumj-core/src/main/java/org/ethereum/vm/DataWord.java b/ethereumj-core/src/main/java/org/ethereum/vm/DataWord.java
index acceac26..33e8de44 100644
--- a/ethereumj-core/src/main/java/org/ethereum/vm/DataWord.java
+++ b/ethereumj-core/src/main/java/org/ethereum/vm/DataWord.java
@@ -295,7 +295,7 @@ public class DataWord implements Comparable<DataWord> {
     }
 
 	public void signExtend(byte k) {
-		if (k > 31)
+		if (0 > k || k > 31)
 			throw new IndexOutOfBoundsException();
 		byte mask = this.sValue().testBit((k * 8) + 7) ? (byte) 0xff : 0;
 		for (int i = 31; i > k; i--) {
diff --git a/ethereumj-core/src/test/java/org/ethereum/vm/DataWordTest.java b/ethereumj-core/src/test/java/org/ethereum/vm/DataWordTest.java
index 5373a0b4..3a8a7063 100644
--- a/ethereumj-core/src/test/java/org/ethereum/vm/DataWordTest.java
+++ b/ethereumj-core/src/test/java/org/ethereum/vm/DataWordTest.java
@@ -216,6 +216,36 @@ public class DataWordTest {
 		System.out.println(x.toString());
 		assertEquals(expected, x.toString());
 	}
+
+	@Test
+	public void testSignExtend8() {
+
+		byte k = 30;
+		DataWord x = new DataWord(Hex.decode("ff34567882345678823456788234567882345678823456788234567882345678"));
+		String expected = "0034567882345678823456788234567882345678823456788234567882345678";
+
+		x.signExtend(k);
+		System.out.println(x.toString());
+		assertEquals(expected, x.toString());
+	}
+
+	@Test(expected=IndexOutOfBoundsException.class)
+	public void testSignExtendException1() {
+
+		byte k = -1;
+		DataWord x = new DataWord();
+
+		x.signExtend(k); // should throw an exception
+	}
+	
+	@Test(expected=IndexOutOfBoundsException.class)
+	public void testSignExtendException2() {
+
+		byte k = 32;
+		DataWord x = new DataWord();
+
+		x.signExtend(k); // should throw an exception
+	}
 	
 	public static BigInteger pow(BigInteger x, BigInteger y) {
 		if (y.compareTo(BigInteger.ZERO) < 0)