diff --git a/buffer/src/main/java/io/netty/buffer/PoolArena.java b/buffer/src/main/java/io/netty/buffer/PoolArena.java
index 348b24f6c8..0a3f9f429e 100644
--- a/buffer/src/main/java/io/netty/buffer/PoolArena.java
+++ b/buffer/src/main/java/io/netty/buffer/PoolArena.java
@@ -233,8 +233,8 @@ abstract class PoolArena<T> {
         allocate(parent.threadCache.get(), buf, newCapacity);
         if (newCapacity > oldCapacity) {
             memoryCopy(
-                    oldMemory, oldOffset + readerIndex,
-                    buf.memory, buf.offset + readerIndex, writerIndex - readerIndex);
+                    oldMemory, oldOffset,
+                    buf.memory, buf.offset, oldCapacity);
         } else if (newCapacity < oldCapacity) {
             if (readerIndex < newCapacity) {
                 if (writerIndex > newCapacity) {
diff --git a/buffer/src/main/java/io/netty/buffer/UnpooledDirectByteBuf.java b/buffer/src/main/java/io/netty/buffer/UnpooledDirectByteBuf.java
index 948b348f24..6661e65bc9 100644
--- a/buffer/src/main/java/io/netty/buffer/UnpooledDirectByteBuf.java
+++ b/buffer/src/main/java/io/netty/buffer/UnpooledDirectByteBuf.java
@@ -141,8 +141,8 @@ public class UnpooledDirectByteBuf extends AbstractReferenceCountedByteBuf {
         if (newCapacity > oldCapacity) {
             ByteBuffer oldBuffer = buffer;
             ByteBuffer newBuffer = ByteBuffer.allocateDirect(newCapacity);
-            oldBuffer.position(readerIndex).limit(writerIndex);
-            newBuffer.position(readerIndex).limit(writerIndex);
+            oldBuffer.position(0).limit(oldBuffer.capacity());
+            newBuffer.position(0).limit(oldBuffer.capacity());
             newBuffer.put(oldBuffer);
             newBuffer.clear();
             setByteBuffer(newBuffer);
diff --git a/buffer/src/main/java/io/netty/buffer/UnpooledHeapByteBuf.java b/buffer/src/main/java/io/netty/buffer/UnpooledHeapByteBuf.java
index 674a7a440c..668b794493 100644
--- a/buffer/src/main/java/io/netty/buffer/UnpooledHeapByteBuf.java
+++ b/buffer/src/main/java/io/netty/buffer/UnpooledHeapByteBuf.java
@@ -112,7 +112,7 @@ public class UnpooledHeapByteBuf extends AbstractReferenceCountedByteBuf {
         int oldCapacity = array.length;
         if (newCapacity > oldCapacity) {
             byte[] newArray = new byte[newCapacity];
-            System.arraycopy(array, readerIndex(), newArray, readerIndex(), readableBytes());
+            System.arraycopy(array, 0, newArray, 0, array.length);
             setArray(newArray);
         } else if (newCapacity < oldCapacity) {
             byte[] newArray = new byte[newCapacity];
diff --git a/buffer/src/main/java/io/netty/buffer/UnpooledUnsafeDirectByteBuf.java b/buffer/src/main/java/io/netty/buffer/UnpooledUnsafeDirectByteBuf.java
index 9d076b09e6..5373bdac21 100644
--- a/buffer/src/main/java/io/netty/buffer/UnpooledUnsafeDirectByteBuf.java
+++ b/buffer/src/main/java/io/netty/buffer/UnpooledUnsafeDirectByteBuf.java
@@ -145,8 +145,8 @@ public class UnpooledUnsafeDirectByteBuf extends AbstractReferenceCountedByteBuf
         if (newCapacity > oldCapacity) {
             ByteBuffer oldBuffer = buffer;
             ByteBuffer newBuffer = ByteBuffer.allocateDirect(newCapacity);
-            oldBuffer.position(readerIndex).limit(writerIndex);
-            newBuffer.position(readerIndex).limit(writerIndex);
+            oldBuffer.position(0).limit(oldBuffer.capacity());
+            newBuffer.position(0).limit(oldBuffer.capacity());
             newBuffer.put(oldBuffer);
             newBuffer.clear();
             setByteBuffer(newBuffer);
diff --git a/buffer/src/test/java/io/netty/buffer/DuplicateByteBufTest.java b/buffer/src/test/java/io/netty/buffer/DuplicateByteBufTest.java
index 684ccbd339..ba957489be 100644
--- a/buffer/src/test/java/io/netty/buffer/DuplicateByteBufTest.java
+++ b/buffer/src/test/java/io/netty/buffer/DuplicateByteBufTest.java
@@ -52,4 +52,15 @@ public class DuplicateByteBufTest extends AbstractByteBufTest {
         super.testInternalNioBuffer();
     }
 
+    // See https://github.com/netty/netty/issues/1800
+    @Test
+    public void testIncreaseCapacityWrapped() {
+        ByteBuf wrapped = buffer.unwrap();
+        wrapped.writeByte(0);
+        wrapped.readerIndex(wrapped.readerIndex() + 1);
+        buffer.writerIndex(buffer.writerIndex() + 1);
+        wrapped.capacity(wrapped.capacity() * 2);
+
+        assertEquals((byte) 0, buffer.readByte());
+    }
 }