diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 6309dfadb..1f31beb1b 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -30,6 +30,10 @@ Improvement::
 * Upgrade to asciidoctorj-diagram 2.2.4 (#1140)
 * Upgrade to jruby 9.3.10.0 (#1138) (@alexlevinfr)
 
+Bug Fixes::
+
+* Fix ConcurrentModificationException when converting to stream concurrently (#1158) (@rocketraman)
+
 Build / Infrastructure::
 
 * Replace use of deprecated 'numbered' attribute by 'sectnums' (#1127) (@abelsromero)
diff --git a/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/JRubyAsciidoctor.java b/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/JRubyAsciidoctor.java
index ff85e1e2b..80e396940 100644
--- a/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/JRubyAsciidoctor.java
+++ b/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/JRubyAsciidoctor.java
@@ -10,8 +10,6 @@
 import org.asciidoctor.extension.JavaExtensionRegistry;
 import org.asciidoctor.extension.RubyExtensionRegistry;
 import org.asciidoctor.jruby.AsciidoctorJRuby;
-import org.asciidoctor.jruby.DirectoryWalker;
-import org.asciidoctor.jruby.ast.impl.AuthorImpl;
 import org.asciidoctor.jruby.ast.impl.DocumentHeaderImpl;
 import org.asciidoctor.jruby.ast.impl.NodeConverter;
 import org.asciidoctor.jruby.converter.internal.ConverterRegistryExecutor;
@@ -23,13 +21,29 @@
 import org.asciidoctor.log.LogHandler;
 import org.asciidoctor.log.LogRecord;
 import org.asciidoctor.syntaxhighlighter.SyntaxHighlighterRegistry;
-import org.jruby.*;
+import org.jruby.Ruby;
+import org.jruby.RubyClass;
+import org.jruby.RubyHash;
+import org.jruby.RubyInstanceConfig;
+import org.jruby.RubyModule;
 import org.jruby.exceptions.RaiseException;
 import org.jruby.javasupport.JavaEmbedUtils;
 import org.jruby.runtime.builtin.IRubyObject;
 
-import java.io.*;
-import java.util.*;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.io.Reader;
+import java.io.Writer;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.UUID;
 import java.util.logging.Logger;
 
 public class JRubyAsciidoctor implements AsciidoctorJRuby, LogHandler {
@@ -60,6 +74,7 @@ private JRubyAsciidoctor(final Ruby rubyRuntime) {
 
         this.rubyGemsPreloader = new RubyGemsPreloader(this.rubyRuntime);
         this.logHandlers.add(new JULLogHandler());
+        RubyOutputStreamWrapper.getOrCreateOutputStreamWrapperClass(this.rubyRuntime);
     }
 
     public static JRubyAsciidoctor create() {
@@ -184,7 +199,6 @@ public DocumentHeader readDocumentHeader(File file) {
         return toDocumentHeader(document);
     }
 
-    @SuppressWarnings("unchecked")
     @Override
     public DocumentHeader readDocumentHeader(String content) {
 
@@ -217,10 +231,6 @@ private List<String> convertAllFiles(Map<String, Object> options, final Iterable
         return asciidoctorContent;
     }
 
-    private List<File> scanForAsciiDocFiles(DirectoryWalker directoryWalker) {
-        return directoryWalker.scan();
-    }
-
     @Override
     public void requireLibrary(String... library) {
         requireLibraries(Arrays.asList(library));
diff --git a/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/RubyOutputStreamWrapper.java b/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/RubyOutputStreamWrapper.java
index 490e3f3db..496bced84 100644
--- a/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/RubyOutputStreamWrapper.java
+++ b/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/RubyOutputStreamWrapper.java
@@ -8,7 +8,6 @@
 import org.jruby.RubyObject;
 import org.jruby.RubyString;
 import org.jruby.anno.JRubyMethod;
-import org.jruby.runtime.ObjectAllocator;
 import org.jruby.runtime.ThreadContext;
 import org.jruby.runtime.builtin.IRubyObject;
 
@@ -25,7 +24,7 @@ public class RubyOutputStreamWrapper extends RubyObject {
 
   public static IRubyObject wrap(final Ruby rubyRuntime, final OutputStream out) {
 
-    final RubyClass rubyClass = getOrCreateOutputStreamWrapperClass(rubyRuntime);
+    final RubyClass rubyClass = getOutputStreamWrapperClass(rubyRuntime);
 
     final IRubyObject wrapper = rubyClass.allocate();
 
@@ -53,18 +52,16 @@ public static RubyClass getOrCreateOutputStreamWrapperClass(final Ruby rubyRunti
       return outputStreamWrapperClass;
     }
 
-    final RubyClass rubyClass = asciidoctorModule.defineClassUnder(RUBY_CLASS_NAME, rubyRuntime.getObject(), new ObjectAllocator() {
-      @Override
-      public IRubyObject allocate(final Ruby runtime, final RubyClass klazz) {
-        return new RubyOutputStreamWrapper(runtime, klazz);
-      }
-    });
-
+    final RubyClass rubyClass = asciidoctorModule.defineClassUnder(RUBY_CLASS_NAME, rubyRuntime.getObject(), RubyOutputStreamWrapper::new);
     rubyClass.defineAnnotatedMethods(RubyOutputStreamWrapper.class);
-
     return rubyClass;
   }
 
+  private static RubyClass getOutputStreamWrapperClass(final Ruby rubyRuntime) {
+    RubyModule asciidoctorModule = rubyRuntime.getModule("AsciidoctorJ");
+    return asciidoctorModule.getClass(RUBY_CLASS_NAME);
+  }
+
   @JRubyMethod(name = "write", required = 1)
   public IRubyObject write(ThreadContext context, IRubyObject arg) throws IOException {
     writeToStream(arg);