diff --git a/VERSION.md b/VERSION.md
index ff3a77f..54ac1f5 100644
--- a/VERSION.md
+++ b/VERSION.md
@@ -1,4 +1,7 @@
-4.5.0
+4.5.1
 
 ## Added
 - Expose structured API error metadata on `FacturapiException`, including `code`, `path`, `location`, `errors`, `logId`, and response headers.
+
+## Fixed
+- Return API error codes as strings, including legacy numeric codes.
diff --git a/src/Exceptions/FacturapiException.php b/src/Exceptions/FacturapiException.php
index 958dadd..ba47d15 100644
--- a/src/Exceptions/FacturapiException.php
+++ b/src/Exceptions/FacturapiException.php
@@ -48,9 +48,22 @@ public function getRawBody(): ?string
 		return $this->rawBody;
 	}
 
-	public function getErrorCode(): mixed
+	public function getErrorCode(): ?string
 	{
-		return is_array($this->errorData) ? ($this->errorData['code'] ?? null) : null;
+		if (!is_array($this->errorData) || !array_key_exists('code', $this->errorData)) {
+			return null;
+		}
+
+		$code = $this->errorData['code'];
+		if ($code === null) {
+			return null;
+		}
+
+		if (is_string($code)) {
+			return $code;
+		}
+
+		return is_int($code) || is_float($code) ? (string) $code : null;
 	}
 
 	public function getErrorPath(): ?string
diff --git a/tests/Http/ErrorHandlingTest.php b/tests/Http/ErrorHandlingTest.php
index d22e7d2..61df2e0 100644
--- a/tests/Http/ErrorHandlingTest.php
+++ b/tests/Http/ErrorHandlingTest.php
@@ -89,4 +89,20 @@ public function testNonJsonErrorsStillExposeRawBody(): void
             self::assertSame($rawBody, $exception->getRawBody());
         }
     }
+
+    public function testNumericApiErrorCodesAreConvertedToStrings(): void
+    {
+        $httpClient = new FakeHttpClient(
+            new Response(400, ['Content-Type' => 'application/json'], '{"code": 400}')
+        );
+
+        $invoices = new Invoices('sk_test_abc123', ['httpClient' => $httpClient]);
+
+        try {
+            $invoices->create(['customer' => []]);
+            self::fail('Expected FacturapiException to be thrown.');
+        } catch (FacturapiException $exception) {
+            self::assertSame('400', $exception->getErrorCode());
+        }
+    }
 }