diff --git a/lib/dispatcher/client-h1.js b/lib/dispatcher/client-h1.js
index e4bcc082e7e..f7149bd6df1 100644
--- a/lib/dispatcher/client-h1.js
+++ b/lib/dispatcher/client-h1.js
@@ -190,7 +190,7 @@ class Parser {
     }
 
     assert(this.ptr != null)
-    assert(currentParser == null)
+    assert(currentParser === null)
 
     this.llhttp.llhttp_resume(this.ptr)
 
@@ -217,22 +217,27 @@ class Parser {
     }
   }
 
-  execute (data) {
+  /**
+   * @param {Buffer} chunk
+   */
+  execute (chunk) {
+    assert(currentParser === null)
     assert(this.ptr != null)
-    assert(currentParser == null)
     assert(!this.paused)
 
     const { socket, llhttp } = this
 
-    if (data.length > currentBufferSize) {
+    // Allocate a new buffer if the current buffer is too small.
+    if (chunk.length > currentBufferSize) {
       if (currentBufferPtr) {
         llhttp.free(currentBufferPtr)
       }
-      currentBufferSize = Math.ceil(data.length / 4096) * 4096
+      // Allocate a buffer that is a multiple of 4096 bytes.
+      currentBufferSize = Math.ceil(chunk.length / 4096) * 4096
       currentBufferPtr = llhttp.malloc(currentBufferSize)
     }
 
-    new Uint8Array(llhttp.memory.buffer, currentBufferPtr, currentBufferSize).set(data)
+    new Uint8Array(llhttp.memory.buffer, currentBufferPtr, currentBufferSize).set(chunk)
 
     // Call `execute` on the wasm parser.
     // We pass the `llhttp_parser` pointer address, the pointer address of buffer view data,
@@ -242,9 +247,9 @@ class Parser {
       let ret
 
       try {
-        currentBufferRef = data
+        currentBufferRef = chunk
         currentParser = this
-        ret = llhttp.llhttp_execute(this.ptr, currentBufferPtr, data.length)
+        ret = llhttp.llhttp_execute(this.ptr, currentBufferPtr, chunk.length)
         /* eslint-disable-next-line no-useless-catch */
       } catch (err) {
         /* istanbul ignore next: difficult to make a test case for */
@@ -254,25 +259,27 @@ class Parser {
         currentBufferRef = null
       }
 
-      const offset = llhttp.llhttp_get_error_pos(this.ptr) - currentBufferPtr
-
-      if (ret === constants.ERROR.PAUSED_UPGRADE) {
-        this.onUpgrade(data.slice(offset))
-      } else if (ret === constants.ERROR.PAUSED) {
-        this.paused = true
-        socket.unshift(data.slice(offset))
-      } else if (ret !== constants.ERROR.OK) {
-        const ptr = llhttp.llhttp_get_error_reason(this.ptr)
-        let message = ''
-        /* istanbul ignore else: difficult to make a test case for */
-        if (ptr) {
-          const len = new Uint8Array(llhttp.memory.buffer, ptr).indexOf(0)
-          message =
-            'Response does not match the HTTP/1.1 protocol (' +
-            Buffer.from(llhttp.memory.buffer, ptr, len).toString() +
-            ')'
+      if (ret !== constants.ERROR.OK) {
+        const data = chunk.subarray(llhttp.llhttp_get_error_pos(this.ptr) - currentBufferPtr)
+
+        if (ret === constants.ERROR.PAUSED_UPGRADE) {
+          this.onUpgrade(data)
+        } else if (ret === constants.ERROR.PAUSED) {
+          this.paused = true
+          socket.unshift(data)
+        } else {
+          const ptr = llhttp.llhttp_get_error_reason(this.ptr)
+          let message = ''
+          /* istanbul ignore else: difficult to make a test case for */
+          if (ptr) {
+            const len = new Uint8Array(llhttp.memory.buffer, ptr).indexOf(0)
+            message =
+              'Response does not match the HTTP/1.1 protocol (' +
+              Buffer.from(llhttp.memory.buffer, ptr, len).toString() +
+              ')'
+          }
+          throw new HTTPParserError(message, constants.ERROR[ret], data)
         }
-        throw new HTTPParserError(message, constants.ERROR[ret], data.slice(offset))
       }
     } catch (err) {
       util.destroy(socket, err)
@@ -280,8 +287,8 @@ class Parser {
   }
 
   destroy () {
+    assert(currentParser === null)
     assert(this.ptr != null)
-    assert(currentParser == null)
 
     this.llhttp.llhttp_free(this.ptr)
     this.ptr = null