diff --git a/lib/zlib.js b/lib/zlib.js
index 4d482addeac8f9..24242ca10113c4 100644
--- a/lib/zlib.js
+++ b/lib/zlib.js
@@ -242,15 +242,11 @@ const FLUSH_BOUND = [
   [ Z_NO_FLUSH, Z_BLOCK ],
   [ BROTLI_OPERATION_PROCESS, BROTLI_OPERATION_EMIT_METADATA ],
 ];
-function isBrotliHandle(handle) {
-  return ((handle instanceof binding.BrotliEncoder) ||
-    (handle instanceof binding.BrotliDecoder)) ? 1 : 0;
-}
+const FLUSH_BOUND_IDX_NORMAL = 0;
+const FLUSH_BOUND_IDX_BROTLI = 1;
 
 // The base class for all Zlib-style streams.
 function ZlibBase(opts, mode, handle, { flush, finishFlush, fullFlush }) {
-  const isBrotli = isBrotliHandle(handle);
-
   let chunkSize = Z_DEFAULT_CHUNK;
   let maxOutputLength = kMaxLength;
   // The ZlibBase class is not exported to user land, the mode should only be
@@ -258,6 +254,13 @@ function ZlibBase(opts, mode, handle, { flush, finishFlush, fullFlush }) {
   assert(typeof mode === 'number');
   assert(mode >= DEFLATE && mode <= BROTLI_ENCODE);
 
+  let flushBoundIdx;
+  if (mode !== BROTLI_ENCODE && mode !== BROTLI_DECODE) {
+    flushBoundIdx = FLUSH_BOUND_IDX_NORMAL;
+  } else {
+    flushBoundIdx = FLUSH_BOUND_IDX_BROTLI;
+  }
+
   if (opts) {
     chunkSize = opts.chunkSize;
     if (!checkFiniteNumber(chunkSize, 'options.chunkSize')) {
@@ -269,11 +272,11 @@ function ZlibBase(opts, mode, handle, { flush, finishFlush, fullFlush }) {
 
     flush = checkRangesOrGetDefault(
       opts.flush, 'options.flush',
-      FLUSH_BOUND[isBrotli][0], FLUSH_BOUND[isBrotli][1], flush);
+      FLUSH_BOUND[flushBoundIdx][0], FLUSH_BOUND[flushBoundIdx][1], flush);
 
     finishFlush = checkRangesOrGetDefault(
       opts.finishFlush, 'options.finishFlush',
-      FLUSH_BOUND[isBrotli][0], FLUSH_BOUND[isBrotli][1], finishFlush);
+      FLUSH_BOUND[flushBoundIdx][0], FLUSH_BOUND[flushBoundIdx][1], finishFlush);
 
     maxOutputLength = checkRangesOrGetDefault(
       opts.maxOutputLength, 'options.maxOutputLength',