diff --git a/dexlib2/src/main/java/org/jf/dexlib2/writer/io/MemoryDataStore.java b/dexlib2/src/main/java/org/jf/dexlib2/writer/io/MemoryDataStore.java
index a3b22d6c..d3c68636 100644
--- a/dexlib2/src/main/java/org/jf/dexlib2/writer/io/MemoryDataStore.java
+++ b/dexlib2/src/main/java/org/jf/dexlib2/writer/io/MemoryDataStore.java
@@ -31,6 +31,7 @@ public class MemoryDataStore implements DexDataStore {
     }
 
     @Nonnull @Override public OutputStream outputAt(final int offset) {
+        if (offset < 0) throw new IllegalArgumentException();
         return new OutputStream() {
             private int position = offset;
             @Override public void write(int b) throws IOException {
@@ -68,6 +69,7 @@ public class MemoryDataStore implements DexDataStore {
     }
 
     @Nonnull @Override public InputStream readAt(final int offset) {
+        if (offset < 0) throw new IllegalArgumentException();
         return new InputStream() {
             private int position = offset;
             private int mark = offset;