diff --git a/CHANGELOG.md b/CHANGELOG.md
index e74cd04..a3b5a75 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
     - Date column decorators `CreateDateColumn`, `UpdateDateColumn` is date by default, `DeleteDateColumn` is nullable,
     - Version column decorator `VersionColumn` is number.
 - Allow lazy relations with `Promise<Relation>` ([#10](https://github.com/daniel7grant/eslint-plugin-typeorm-typescript/issues/10))
+- Report error when the relation doesn't have an arrow function defined
 
 ### Changed
 
diff --git a/src/rules/enforce-relation-types.test.ts b/src/rules/enforce-relation-types.test.ts
index 80ef265..cf14723 100644
--- a/src/rules/enforce-relation-types.test.ts
+++ b/src/rules/enforce-relation-types.test.ts
@@ -135,6 +135,19 @@ ruleTester.run('enforce-relation-types', enforceRelationTypes, {
         },
     ],
     invalid: [
+        {
+            name: 'should fail on nullable one-to-one relations',
+            code: `class Entity {
+                @OneToOne()
+                @JoinColumn()
+                other: Other;
+            }`,
+            errors: [
+                {
+                    messageId: 'typescript_typeorm_relation_missing',
+                },
+            ],
+        },
         {
             name: 'should fail on nullable one-to-one relations',
             code: `class Entity {
diff --git a/src/rules/enforce-relation-types.ts b/src/rules/enforce-relation-types.ts
index dea2ddf..bb5e9b3 100644
--- a/src/rules/enforce-relation-types.ts
+++ b/src/rules/enforce-relation-types.ts
@@ -1,4 +1,4 @@
-import { AST_NODE_TYPES, ESLintUtils, TSESTree } from '@typescript-eslint/utils';
+import { AST_NODE_TYPES, ESLintUtils } from '@typescript-eslint/utils';
 import { ReportSuggestionArray } from '@typescript-eslint/utils/ts-eslint';
 import {
     findEitherDecoratorArguments,
@@ -20,6 +20,7 @@ const createRule = ESLintUtils.RuleCreator(
 );
 
 type EnforceColumnMessages =
+    | 'typescript_typeorm_relation_missing'
     | 'typescript_typeorm_relation_mismatch'
     | 'typescript_typeorm_relation_array_to_many'
     | 'typescript_typeorm_relation_nullable_by_default'
@@ -37,6 +38,8 @@ const enforceColumnTypes = createRule({
         },
         hasSuggestions: true,
         messages: {
+            typescript_typeorm_relation_missing:
+                'Relation {{ relation }} of {{ propertyName }}{{ className }} does not have an arrow function with the relation type.',
             typescript_typeorm_relation_mismatch:
                 'Type of {{ propertyName }}{{ className }} is not consistent with the TypeORM relation type {{ relation }}{{ expectedValue }}.',
             typescript_typeorm_relation_array_to_many:
@@ -66,7 +69,21 @@ const enforceColumnTypes = createRule({
                 const [relation, relArguments] = relationArguments;
                 const typeormType = convertArgumentToRelationType(relation, relArguments);
                 if (!typeormType) {
-                    return; // TODO: report error
+                    const propertyName =
+                        node.key?.type === AST_NODE_TYPES.Identifier ? node.key.name : 'property';
+                    const classObject = findParentClass(node);
+                    const className = classObject?.id ? ` in ${classObject.id.name}` : '';
+                    context.report({
+                        node,
+                        messageId: 'typescript_typeorm_relation_missing',
+                        data: {
+                            className,
+                            propertyName,
+                            relation,
+                        },
+                        loc: node.loc,
+                    });
+                    return;
                 }
 
                 if (!node.typeAnnotation) {