diff --git a/core/src/main/java/org/teavm/backend/c/CTarget.java b/core/src/main/java/org/teavm/backend/c/CTarget.java
index 5bf0c2134..3716eeb72 100644
--- a/core/src/main/java/org/teavm/backend/c/CTarget.java
+++ b/core/src/main/java/org/teavm/backend/c/CTarget.java
@@ -152,6 +152,7 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
     private List<GeneratorFactory> generatorFactories = new ArrayList<>();
     private Characteristics characteristics;
     private Set<MethodReference> asyncMethods;
+    private boolean hasThreads;
     private MethodNodeCache astCache = EmptyMethodNodeCache.INSTANCE;
     private boolean incremental;
     private boolean lineNumbersGenerated;
@@ -285,11 +286,11 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
 
     @Override
     public void analyzeBeforeOptimizations(ListableClassReaderSource classSource) {
-        AsyncMethodFinder asyncFinder = new AsyncMethodFinder(controller.getDependencyInfo().getCallGraph(),
-                controller.getDiagnostics());
+        AsyncMethodFinder asyncFinder = new AsyncMethodFinder(controller.getDependencyInfo().getCallGraph());
         asyncFinder.find(classSource);
         asyncMethods = new HashSet<>(asyncFinder.getAsyncMethods());
         asyncMethods.addAll(asyncFinder.getAsyncFamilyMethods());
+        hasThreads = asyncFinder.hasAsyncMethods();
     }
 
     @Override
@@ -302,7 +303,7 @@ public class CTarget implements TeaVMTarget, TeaVMCHost {
         classInitializerEliminator.apply(program);
         classInitializerTransformer.transform(program);
         nullCheckTransformation.apply(program, method.getResultType());
-        new CoroutineTransformation(controller.getUnprocessedClassSource(), asyncMethods)
+        new CoroutineTransformation(controller.getUnprocessedClassSource(), asyncMethods, hasThreads)
                 .apply(program, method.getReference());
         ShadowStackTransformer shadowStackTransformer = !incremental
                 ? this.shadowStackTransformer
diff --git a/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java b/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
index 4f640dc01..0bf35c72c 100644
--- a/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
+++ b/core/src/main/java/org/teavm/backend/javascript/JavaScriptTarget.java
@@ -470,8 +470,7 @@ public class JavaScriptTarget implements TeaVMTarget, TeaVMJavaScriptHost {
     }
 
     private List<PreparedClass> modelToAst(ListableClassHolderSource classes) {
-        AsyncMethodFinder asyncFinder = new AsyncMethodFinder(controller.getDependencyInfo().getCallGraph(),
-                controller.getDiagnostics());
+        AsyncMethodFinder asyncFinder = new AsyncMethodFinder(controller.getDependencyInfo().getCallGraph());
         asyncFinder.find(classes);
         asyncMethods.addAll(asyncFinder.getAsyncMethods());
         asyncFamilyMethods.addAll(asyncFinder.getAsyncFamilyMethods());
diff --git a/core/src/main/java/org/teavm/backend/lowlevel/transform/CoroutineTransformation.java b/core/src/main/java/org/teavm/backend/lowlevel/transform/CoroutineTransformation.java
index 796e4daa4..6688a373e 100644
--- a/core/src/main/java/org/teavm/backend/lowlevel/transform/CoroutineTransformation.java
+++ b/core/src/main/java/org/teavm/backend/lowlevel/transform/CoroutineTransformation.java
@@ -79,10 +79,13 @@ public class CoroutineTransformation {
     private SwitchInstruction resumeSwitch;
     private int parameterCount;
     private ValueType returnType;
+    private boolean hasThreads;
 
-    public CoroutineTransformation(ClassReaderSource classSource, Set<MethodReference> asyncMethods) {
+    public CoroutineTransformation(ClassReaderSource classSource, Set<MethodReference> asyncMethods,
+            boolean hasThreads) {
         this.classSource = classSource;
         this.asyncMethods = asyncMethods;
+        this.hasThreads = hasThreads;
     }
 
     public void apply(Program program, MethodReference methodReference) {
@@ -233,7 +236,7 @@ public class CoroutineTransformation {
         } else if (instruction instanceof InitClassInstruction) {
             return isSplittingClassInitializer(((InitClassInstruction) instruction).getClassName());
         } else {
-            return instruction instanceof MonitorEnterInstruction;
+            return hasThreads && instruction instanceof MonitorEnterInstruction;
         }
     }
 
diff --git a/core/src/main/java/org/teavm/model/util/AsyncMethodFinder.java b/core/src/main/java/org/teavm/model/util/AsyncMethodFinder.java
index 02cd66156..7299aba07 100644
--- a/core/src/main/java/org/teavm/model/util/AsyncMethodFinder.java
+++ b/core/src/main/java/org/teavm/model/util/AsyncMethodFinder.java
@@ -25,7 +25,6 @@ import java.util.Set;
 import org.teavm.callgraph.CallGraph;
 import org.teavm.callgraph.CallGraphNode;
 import org.teavm.callgraph.CallSite;
-import org.teavm.diagnostics.Diagnostics;
 import org.teavm.interop.Async;
 import org.teavm.model.ClassReader;
 import org.teavm.model.ElementModifier;
@@ -44,13 +43,11 @@ public class AsyncMethodFinder {
     private Set<MethodReference> readonlyAsyncMethods = Collections.unmodifiableSet(asyncMethods);
     private Set<MethodReference> readonlyAsyncFamilyMethods = Collections.unmodifiableSet(asyncFamilyMethods.keySet());
     private CallGraph callGraph;
-    private Diagnostics diagnostics;
     private ListableClassReaderSource classSource;
     private boolean hasAsyncMethods;
 
-    public AsyncMethodFinder(CallGraph callGraph, Diagnostics diagnostics) {
+    public AsyncMethodFinder(CallGraph callGraph) {
         this.callGraph = callGraph;
-        this.diagnostics = diagnostics;
     }
 
     public Set<MethodReference> getAsyncMethods() {
@@ -63,7 +60,7 @@ public class AsyncMethodFinder {
 
     public void find(ListableClassReaderSource classSource) {
         this.classSource = classSource;
-        hasAsyncMethods = hasAsyncMethods();
+        hasAsyncMethods = findAsyncMethods();
         for (String clsName : classSource.getClassNames()) {
             ClassReader cls = classSource.get(clsName);
             for (MethodReader method : cls.getMethods()) {
@@ -104,7 +101,7 @@ public class AsyncMethodFinder {
         }
     }
 
-    private boolean hasAsyncMethods() {
+    private boolean findAsyncMethods() {
         boolean result = false;
         loop: for (String clsName : classSource.getClassNames()) {
             ClassReader cls = classSource.get(clsName);
@@ -123,6 +120,10 @@ public class AsyncMethodFinder {
         return result && method != null;
     }
 
+    public boolean hasAsyncMethods() {
+        return hasAsyncMethods;
+    }
+
     private boolean hasMonitor(MethodReader method) {
         if (method.hasModifier(ElementModifier.SYNCHRONIZED)) {
             return true;