diff --git a/hpack/shared/src/main/scala/org/http4s/hpack/Decoder.scala b/hpack/shared/src/main/scala/org/http4s/hpack/Decoder.scala
index e522ed7..499480f 100644
--- a/hpack/shared/src/main/scala/org/http4s/hpack/Decoder.scala
+++ b/hpack/shared/src/main/scala/org/http4s/hpack/Decoder.scala
@@ -31,12 +31,11 @@
  */
 package org.http4s.hpack;
 
-import java.io.IOException;
-import java.io.InputStream;
-
-import org.http4s.hpack.HpackUtil.IndexType;
+import org.http4s.hpack.HeaderField.HEADER_ENTRY_OVERHEAD
+import org.http4s.hpack.HpackUtil.IndexType
 
-import org.http4s.hpack.HeaderField.HEADER_ENTRY_OVERHEAD;
+import java.io.IOException
+import java.io.InputStream;
 
 private[http4s] final class Decoder(dynamicTable: DynamicTable) {
 
@@ -48,6 +47,11 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
     new IOException("invalid max dynamic table size");
   private[this] val MAX_DYNAMIC_TABLE_SIZE_CHANGE_REQUIRED =
     new IOException("max dynamic table size change required");
+  private[this] val UNEXPECTED_DYNAMIC_TABLE_SIZE_UPDATE =
+    new IOException(
+      "Dynamic table size update must happen "
+        + "at the beginning of the header block"
+    )
 
   private[this] val EMPTY = Array.emptyByteArray;
 
@@ -103,6 +107,8 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
     */
   @throws[IOException]
   def decode(in: InputStream, headerListener: HeaderListener): Unit = {
+    var seenOnlyDynamicTableSizeUpdate = true
+
     while (in.available() > 0) {
       var b: Byte = 0
       import State._
@@ -115,6 +121,7 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
           }
           if (b < 0) {
             // Indexed Header Field
+            seenOnlyDynamicTableSizeUpdate = false
             index = b & 0x7f;
             if (index == 0) {
               throw ILLEGAL_INDEX_VALUE;
@@ -125,6 +132,7 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
             }
           } else if ((b & 0x40) == 0x40) {
             // Literal Header Field with Incremental Indexing
+            seenOnlyDynamicTableSizeUpdate = false
             indexType = IndexType.INCREMENTAL;
             index = b & 0x3f;
             if (index == 0) {
@@ -138,15 +146,20 @@ private[http4s] final class Decoder(dynamicTable: DynamicTable) {
             }
           } else if ((b & 0x20) == 0x20) {
             // Dynamic Table Size Update
-            index = b & 0x1f;
-            if (index == 0x1f) {
-              state = State.READ_MAX_DYNAMIC_TABLE_SIZE;
+            if (seenOnlyDynamicTableSizeUpdate) {
+              index = b & 0x1f;
+              if (index == 0x1f) {
+                state = State.READ_MAX_DYNAMIC_TABLE_SIZE;
+              } else {
+                setDynamicTableSize(index);
+                state = State.READ_HEADER_REPRESENTATION;
+              }
             } else {
-              setDynamicTableSize(index);
-              state = State.READ_HEADER_REPRESENTATION;
+              throw UNEXPECTED_DYNAMIC_TABLE_SIZE_UPDATE
             }
           } else {
             // Literal Header Field without Indexing / never Indexed
+            seenOnlyDynamicTableSizeUpdate = false
             indexType = if ((b & 0x10) == 0x10) IndexType.NEVER else IndexType.NONE;
             index = b & 0x0f;
             if (index == 0) {
diff --git a/hpack/shared/src/test/scala/org/http4s/hpack/DecoderTest.scala b/hpack/shared/src/test/scala/org/http4s/hpack/DecoderTest.scala
index 921c16c..29ac698 100644
--- a/hpack/shared/src/test/scala/org/http4s/hpack/DecoderTest.scala
+++ b/hpack/shared/src/test/scala/org/http4s/hpack/DecoderTest.scala
@@ -33,15 +33,15 @@ package org.http4s.hpack
 
 import cats.kernel.Eq
 import cats.syntax.all._
-
-import java.io.ByteArrayInputStream
-import java.io.IOException
-import org.junit.Before
-import org.junit.Test
 import org.http4s.hpack.HpackUtil.ISO_8859_1
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
 import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+
+import java.io.ByteArrayInputStream
+import java.io.IOException
 
 object DecoderTest {
   private val MAX_HEADER_SIZE = 8192
@@ -142,6 +142,16 @@ class DecoderTest {
     decode("2081")
   }
 
+  @Test(expected = classOf[IOException])
+  @throws[IOException]
+  def testDynamicTableSizeUpdateAfterTheBeginningOfTheBlock(): Unit =
+    decode("8120")
+
+  @Test(expected = classOf[IOException])
+  @throws[Exception]
+  def testDynamicTableSizeUpdateAfterTheBeginningOfTheBlockLong(): Unit =
+    decode("813FE11F")
+
   @Test(expected = classOf[IOException])
   @throws[Exception]
   def testTooLargeDynamicTableSizeUpdate(): Unit = {