diff --git a/src/ApiResult.php b/src/ApiResult.php
index 4a5e274..0360017 100644
--- a/src/ApiResult.php
+++ b/src/ApiResult.php
@@ -36,13 +36,13 @@ public function getError() : ?string {
         // formatted API error
         $error = $this->getVal('body/error/message');
         if($error !== null) {
-            return $error;
+            return StringEx::stringify($error);
         }
 
         // exception API error
         $error = $this->getVal('body/exception/message');
         if($error !== null) {
-            return $error;
+            return StringEx::stringify($error);
         }
 
         // curl error
@@ -57,7 +57,7 @@ public function getError() : ?string {
             if(is_array($error)) {
                 return $this->getXml('body');
             }
-            return $error;
+            return StringEx::stringify($error);
         }
         return null;
     }
diff --git a/tests/ApiResult/getError_Test.php b/tests/ApiResult/getError_Test.php
index 6feaa29..32bd30b 100644
--- a/tests/ApiResult/getError_Test.php
+++ b/tests/ApiResult/getError_Test.php
@@ -28,7 +28,14 @@ public static function body_expected_dataProvider() : array {
             [['error' => ['message' => 'foo']], 'foo'],
             [['exception' => ['message' => 'bar']], 'bar'],
             [['foo' => ['qux' => ['@id' => '123']]], '<foo><qux id="123"></qux></foo>'],
-            ['<error>qux</error>', '<error>qux</error>']
+            ['<error>qux</error>', '<error>qux</error>'],
+
+            // non-string scalars deserialized from the API response body must be coerced to
+            // string, not returned as-is: getError() is declared ": ?string" and under
+            // strict_types this throws a TypeError instead of returning a value
+            [['error' => ['message' => true]], '1'],
+            [['exception' => ['message' => 42]], '42'],
+            [3.14, '3.14']
         ];
     }
 
@@ -36,7 +43,7 @@ public static function body_expected_dataProvider() : array {
      * @dataProvider body_expected_dataProvider
      * @test
      */
-    public function Can_get_error_from_body(string|array $body, string $expected) {
+    public function Can_get_error_from_body(string|array|bool|int|float $body, string $expected) {
 
         // arrange
         $data = [