diff --git a/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionEncoder.java b/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionEncoder.java
index 6ee351c7eb..2bc22a422c 100644
--- a/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionEncoder.java
+++ b/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionEncoder.java
@@ -331,22 +331,26 @@ public class DefaultHttp2ConnectionEncoder implements Http2ConnectionEncoder {
      */
     private final class FlowControlledData extends FlowControlledBase {
         private final CoalescingBufferQueue queue;
+        private int dataSize;
 
         public FlowControlledData(Http2Stream stream, ByteBuf buf, int padding, boolean endOfStream,
                                    ChannelPromise promise) {
             super(stream, padding, endOfStream, promise);
             queue = new CoalescingBufferQueue(promise.channel());
             queue.add(buf, promise);
+            dataSize = queue.readableBytes();
         }
 
         @Override
         public int size() {
-            return queue.readableBytes() + padding;
+            return dataSize + padding;
         }
 
         @Override
         public void error(ChannelHandlerContext ctx, Throwable cause) {
             queue.releaseAndFailAll(cause);
+            // Don't update dataSize because we need to ensure the size() method returns a consistent size even after
+            // error so we don't invalidate flow control when returning bytes to flow control.
             lifecycleManager.onError(ctx, cause);
             promise.tryFailure(cause);
         }
@@ -363,6 +367,7 @@ public class DefaultHttp2ConnectionEncoder implements Http2ConnectionEncoder {
             int writeableData = min(queuedData, allowedBytes);
             ChannelPromise writePromise = ctx.newPromise().addListener(this);
             ByteBuf toWrite = queue.remove(writeableData, writePromise);
+            dataSize = queue.readableBytes();
 
             // Determine how much padding to write.
             int writeablePadding = min(allowedBytes - writeableData, padding);
@@ -381,6 +386,7 @@ public class DefaultHttp2ConnectionEncoder implements Http2ConnectionEncoder {
                 return false;
             }
             nextData.queue.copyTo(queue);
+            dataSize = queue.readableBytes();
             // Given that we're merging data into a frame it doesn't really make sense to accumulate padding.
             padding = Math.max(padding, nextData.padding);
             endOfStream = nextData.endOfStream;
diff --git a/codec-http2/src/main/java/io/netty/handler/codec/http2/UniformStreamByteDistributor.java b/codec-http2/src/main/java/io/netty/handler/codec/http2/UniformStreamByteDistributor.java
index 565ddfaa26..42b864ce19 100644
--- a/codec-http2/src/main/java/io/netty/handler/codec/http2/UniformStreamByteDistributor.java
+++ b/codec-http2/src/main/java/io/netty/handler/codec/http2/UniformStreamByteDistributor.java
@@ -146,7 +146,8 @@ public final class UniformStreamByteDistributor implements StreamByteDistributor
         }
 
         void updateStreamableBytes(int newStreamableBytes, boolean hasFrame, int windowSize) {
-            assert hasFrame || newStreamableBytes == 0;
+            assert hasFrame || newStreamableBytes == 0 :
+                "hasFrame: " + hasFrame + " newStreamableBytes: " + newStreamableBytes;
 
             int delta = newStreamableBytes - streamableBytes;
             if (delta != 0) {
diff --git a/transport/src/main/java/io/netty/channel/CoalescingBufferQueue.java b/transport/src/main/java/io/netty/channel/CoalescingBufferQueue.java
index 6512103d52..39a88ab576 100644
--- a/transport/src/main/java/io/netty/channel/CoalescingBufferQueue.java
+++ b/transport/src/main/java/io/netty/channel/CoalescingBufferQueue.java
@@ -37,11 +37,16 @@ import java.util.ArrayDeque;
 public final class CoalescingBufferQueue {
 
     private final Channel channel;
-    private final ArrayDeque<Object> bufAndListenerPairs = new ArrayDeque<Object>();
+    private final ArrayDeque<Object> bufAndListenerPairs;
     private int readableBytes;
 
     public CoalescingBufferQueue(Channel channel) {
+        this(channel, 4);
+    }
+
+    public CoalescingBufferQueue(Channel channel, int initSize) {
         this.channel = ObjectUtil.checkNotNull(channel, "channel");
+        bufAndListenerPairs = new ArrayDeque<Object>(initSize);
     }
 
     /**