diff --git a/src/de/inetsoftware/jwebassembly/module/WasmLoadStoreInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmLoadStoreInstruction.java
index 0bf2990..169a633 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmLoadStoreInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmLoadStoreInstruction.java
@@ -1,5 +1,5 @@
 /*
-   Copyright 2018 - 2020 Volker Berlin (i-net software)
+   Copyright 2018 - 2021 Volker Berlin (i-net software)
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -43,7 +43,7 @@ class WasmLoadStoreInstruction extends WasmLocalInstruction {
      * @param lineNumber
      *            the line number in the Java source code
      */
-    WasmLoadStoreInstruction( VariableOperator op, @Nonnegative int idx, LocaleVariableManager localVariables, int javaCodePos, int lineNumber ) {
+    WasmLoadStoreInstruction( @Nonnull VariableOperator op, @Nonnegative int idx, LocaleVariableManager localVariables, int javaCodePos, int lineNumber ) {
         super( op, idx, localVariables, javaCodePos, lineNumber );
     }
 
diff --git a/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java b/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
index a82216a..df4c51f 100644
--- a/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
+++ b/src/de/inetsoftware/jwebassembly/module/WasmLocalInstruction.java
@@ -34,6 +34,7 @@ import static de.inetsoftware.jwebassembly.wasm.VariableOperator.*;
  */
 class WasmLocalInstruction extends WasmInstruction {
 
+    @Nonnull
     private VariableOperator      op;
 
     private int                   idx;
@@ -54,7 +55,7 @@ class WasmLocalInstruction extends WasmInstruction {
      * @param lineNumber
      *            the line number in the Java source code
      */
-    WasmLocalInstruction( VariableOperator op, @Nonnegative int idx, LocaleVariableManager localVariables, int javaCodePos, int lineNumber ) {
+    WasmLocalInstruction( @Nonnull VariableOperator op, @Nonnegative int idx, LocaleVariableManager localVariables, int javaCodePos, int lineNumber ) {
         super( javaCodePos, lineNumber );
         this.op = op;
         this.idx = idx;
@@ -74,6 +75,7 @@ class WasmLocalInstruction extends WasmInstruction {
      * 
      * @return the operator
      */
+    @Nonnull
     VariableOperator getOperator() {
         return op;
     }
@@ -84,7 +86,7 @@ class WasmLocalInstruction extends WasmInstruction {
      * @param op
      *            the operator
      */
-    void setOperator( VariableOperator op ) {
+    void setOperator( @Nonnull VariableOperator op ) {
         this.op = op;
     }