diff --git a/src/Schema/Keywords/Nullable.php b/src/Schema/Keywords/Nullable.php
index 9569ac27..68c4350f 100644
--- a/src/Schema/Keywords/Nullable.php
+++ b/src/Schema/Keywords/Nullable.php
@@ -7,7 +7,7 @@
 use League\OpenAPIValidation\Schema\Exception\KeywordMismatch;
 
 use function in_array;
-use function is_string;
+use function is_array;
 
 class Nullable extends BaseKeyword
 {
@@ -27,6 +27,6 @@ public function validate($data, bool $nullable): void
 
     public function nullableByType(): bool
     {
-        return ! is_string($this->parentSchema->type) && in_array('null', $this->parentSchema->type);
+        return is_array($this->parentSchema->type) && in_array('null', $this->parentSchema->type);
     }
 }
diff --git a/tests/FromCommunity/IssueNullablePropertyTest.php b/tests/FromCommunity/IssueNullablePropertyTest.php
new file mode 100644
index 00000000..3086e36c
--- /dev/null
+++ b/tests/FromCommunity/IssueNullablePropertyTest.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace League\OpenAPIValidation\Tests\FromCommunity;
+
+use GuzzleHttp\Psr7\Request;
+use League\OpenAPIValidation\PSR7\Exception\Validation\InvalidBody;
+use League\OpenAPIValidation\PSR7\ValidatorBuilder;
+use League\OpenAPIValidation\Tests\PSR7\BaseValidatorTest;
+
+use function json_encode;
+
+final class IssueNullablePropertyTest extends BaseValidatorTest
+{
+    public function testNullableFalseThrowInvalidBody(): void
+    {
+        $json = /** @lang json */
+        <<<JSON
+{
+  "openapi": "3.0.0",
+  "paths": {
+    "/api/data": {
+      "post": {
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "type": "object",
+                "required": [
+                  "value"
+                ],
+                "properties": {
+                  "value": {
+                    "nullable": false,
+                    "oneOf": [
+                      {"type": "string"},
+                      {"type": "boolean"}
+                    ]
+                  }
+                }
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "A response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "string"
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}
+JSON;
+
+        $requestValidator = (new ValidatorBuilder())
+            ->fromJson($json)
+            ->getRequestValidator();
+
+        $request = new Request(
+            'POST',
+            'headers:/api/data',
+            ['Content-Type' => 'application/json'],
+            json_encode(['value' => null])
+        );
+
+        $this->expectException(InvalidBody::class);
+
+        $requestValidator->validate($request);
+    }
+}