diff --git a/base/src/main/java/org/arend/typechecking/visitor/CheckTypeVisitor.java b/base/src/main/java/org/arend/typechecking/visitor/CheckTypeVisitor.java
index 9b81ce7d4..3eb5594b8 100644
--- a/base/src/main/java/org/arend/typechecking/visitor/CheckTypeVisitor.java
+++ b/base/src/main/java/org/arend/typechecking/visitor/CheckTypeVisitor.java
@@ -1333,20 +1333,25 @@ private TypecheckingResult typecheckClassExt(List<? extends Concrete.ClassFieldI
     for (Concrete.ClassFieldImpl classFieldImpl : classFieldImpls) {
       Definition definition = referableToDefinition(classFieldImpl.getImplementedField(), classFieldImpl);
       if (definition != null) {
-        implementations.add(new Pair<>(definition,classFieldImpl));
+        boolean ok = true;
         if (definition instanceof ClassField) {
           if (baseClass.getFields().contains(definition)) {
             defined.add((ClassField) definition);
           } else {
             errorReporter.report(new IncorrectImplementationError((ClassField) definition, baseClass, classFieldImpl));
+            ok = false;
           }
         } else if (definition instanceof ClassDefinition) {
           if (baseClass.isSubClassOf((ClassDefinition) definition)) {
             defined.addAll(((ClassDefinition) definition).getFields());
           } else {
             errorReporter.report(new IncorrectImplementationError((ClassDefinition) definition, baseClass, classFieldImpl));
+            ok = false;
           }
         }
+        if (ok) {
+          implementations.add(new Pair<>(definition, classFieldImpl));
+        }
       }
     }