diff --git a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/components/Expression.java b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/components/Expression.java
index 23aab80..6e7f775 100644
--- a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/components/Expression.java
+++ b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/components/Expression.java
@@ -234,6 +234,15 @@ public static TypeTestExpressionNode getTypeTestExpressionNode(ExpressionNode ex
         );
     }
 
+    public static TypeTestExpressionNode getUnaryTypeTestExpressionNode(ExpressionNode expression,
+                                                                        Node typeDescriptor) {
+        return NodeFactory.createTypeTestExpressionNode(
+                expression,
+                SyntaxTreeConstants.SYNTAX_TREE_KEYWORD_NOT_IS,
+                typeDescriptor
+        );
+    }
+
     public static CheckExpressionNode getCheckExpressionNode(ExpressionNode expression) {
         return NodeFactory.createCheckExpressionNode(
                 SyntaxKind.CHECK_ACTION,
diff --git a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/constants/SyntaxTreeConstants.java b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/constants/SyntaxTreeConstants.java
index 80a88a0..d477725 100644
--- a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/constants/SyntaxTreeConstants.java
+++ b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/constants/SyntaxTreeConstants.java
@@ -61,7 +61,6 @@ private SyntaxTreeConstants() {
     public static final Token SYNTAX_TREE_UNDERSCORE = AbstractNodeFactory.createIdentifierToken("_");
     public static final Token SYNTAX_TREE_OPTIONAL_CHAINING = AbstractNodeFactory.createIdentifierToken("?.");
     public static final Token SYNTAX_TREE_AT = AbstractNodeFactory.createIdentifierToken("@");
-
     public static final Token SYNTAX_TREE_KEYWORD_NEW = AbstractNodeFactory.createIdentifierToken("new ");
     public static final Token SYNTAX_TREE_KEYWORD_IMPORT = AbstractNodeFactory.createIdentifierToken("import ");
     public static final Token SYNTAX_TREE_KEYWORD_CHECK = AbstractNodeFactory.createIdentifierToken("check ");
@@ -74,6 +73,7 @@ private SyntaxTreeConstants() {
     public static final Token SYNTAX_TREE_KEYWORD_IF = AbstractNodeFactory.createIdentifierToken("if ");
     public static final Token SYNTAX_TREE_KEYWORD_ELSE = AbstractNodeFactory.createIdentifierToken("else ");
     public static final Token SYNTAX_TREE_KEYWORD_IS = AbstractNodeFactory.createIdentifierToken("is ");
+    public static final Token SYNTAX_TREE_KEYWORD_NOT_IS = AbstractNodeFactory.createIdentifierToken("!is");
     public static final Token SYNTAX_TREE_KEYWORD_LISTENER = AbstractNodeFactory.createIdentifierToken("listener ");
     public static final Token SYNTAX_TREE_KEYWORD_SERVICE = AbstractNodeFactory.createIdentifierToken("service ");
     public static final Token SYNTAX_TREE_KEYWORD_ON = AbstractNodeFactory.createIdentifierToken("on ");
diff --git a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/utils/MessageUtils.java b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/utils/MessageUtils.java
index 873d0e2..f2753fc 100644
--- a/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/utils/MessageUtils.java
+++ b/protoc-cli/src/main/java/io/ballerina/protoc/builder/syntaxtree/utils/MessageUtils.java
@@ -46,8 +46,7 @@
 import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getFieldAccessExpressionNode;
 import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getMethodCallExpressionNode;
 import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getOptionalFieldAccessExpressionNode;
-import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getTypeTestExpressionNode;
-import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getUnaryExpressionNode;
+import static io.ballerina.protoc.builder.syntaxtree.components.Expression.getUnaryTypeTestExpressionNode;
 import static io.ballerina.protoc.builder.syntaxtree.components.Literal.getBooleanLiteralNode;
 import static io.ballerina.protoc.builder.syntaxtree.components.Literal.getNumericLiteralNode;
 import static io.ballerina.protoc.builder.syntaxtree.components.Statement.getCompoundAssignmentStatementNode;
@@ -247,14 +246,10 @@ private static Function getValidationFunction(Message message) {
             function.addVariableStatement(count.getVariableDeclarationNode());
             for (Field field : oneOfFieldMap.getValue()) {
                 IfElse oneOfFieldCheck = new IfElse(
-                        getUnaryExpressionNode(
-                                getBracedExpressionNode(
-                                        getTypeTestExpressionNode(
-                                                getOptionalFieldAccessExpressionNode("r", field.getFieldName()),
-                                                getNilTypeDescriptorNode()
-                                        )
-                                )
-                        )
+                    getUnaryTypeTestExpressionNode(
+                            getOptionalFieldAccessExpressionNode("r", field.getFieldName()),
+                            getNilTypeDescriptorNode()
+                    )
                 );
                 oneOfFieldCheck.addIfStatement(
                         getCompoundAssignmentStatementNode(