diff --git a/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java b/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
index 280617112..25f892c17 100644
--- a/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
+++ b/core/src/main/java/org/teavm/ast/optimization/OptimizingVisitor.java
@@ -246,7 +246,7 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
         pushLocation(expr.getLocation());
         try {
             expr.getCondition().acceptVisitor(this);
-            Expr cond = resultExpr;
+            Expr cond = optimizeCondition(resultExpr);
 
             Statement barrier = addBarrier();
             expr.getConsequent().acceptVisitor(this);
@@ -786,7 +786,7 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
     @Override
     public void visit(ConditionalStatement statement) {
         statement.getCondition().acceptVisitor(this);
-        statement.setCondition(resultExpr);
+        statement.setCondition(optimizeCondition(resultExpr));
         List<Statement> consequent = processSequence(statement.getConsequent());
         List<Statement> alternative = processSequence(statement.getAlternative());
         if (consequent.isEmpty()) {
@@ -871,7 +871,14 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
         List<Statement> newDefault = processSequence(statement.getDefaultClause());
         statement.getDefaultClause().clear();
         statement.getDefaultClause().addAll(newDefault);
-        resultStmt = statement;
+
+        if (statement.getClauses().isEmpty()) {
+            SequentialStatement seq = new SequentialStatement();
+            seq.getSequence().addAll(statement.getDefaultClause());
+            resultStmt = seq;
+        } else {
+            resultStmt = statement;
+        }
     }
 
     @Override
@@ -1074,4 +1081,19 @@ class OptimizingVisitor implements StatementVisitor, ExprVisitor {
 
         return false;
     }
+
+    private Expr optimizeCondition(Expr expr) {
+        if (expr instanceof BinaryExpr) {
+            BinaryExpr binary = (BinaryExpr) expr;
+            if (isZero(((BinaryExpr) expr).getSecondOperand())) {
+                switch (binary.getOperation()) {
+                    case EQUALS:
+                        return ExprOptimizer.invert(binary.getFirstOperand());
+                    case NOT_EQUALS:
+                        return binary.getFirstOperand();
+                }
+            }
+        }
+        return expr;
+    }
 }
diff --git a/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java b/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
index f823c1677..5a85eb571 100644
--- a/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
+++ b/core/src/main/java/org/teavm/backend/javascript/rendering/StatementRenderer.java
@@ -1354,6 +1354,9 @@ public class StatementRenderer implements ExprVisitor, StatementVisitor {
                     precedence = Precedence.CONDITIONAL.next();
                     expr.getExpr().acceptVisitor(this);
                     writer.append(" instanceof ").appendClass(clsName);
+                    if (needsParentheses) {
+                        writer.append(')');
+                    }
                     if (expr.getLocation() != null) {
                         popLocation();
                     }