diff --git a/src/LaunchDarkly/Impl/Util.php b/src/LaunchDarkly/Impl/Util.php
index 05649307..0cfcefa0 100644
--- a/src/LaunchDarkly/Impl/Util.php
+++ b/src/LaunchDarkly/Impl/Util.php
@@ -138,7 +138,7 @@ public static function defaultHeaders(string $sdkKey, array $options): array
     public static function eventHeaders(string $sdkKey, array $options): array
     {
         $headers = Util::defaultHeaders($sdkKey, $options);
-        $headers['X-LaunchDarkly-Event-Schema'] = EventPublisher::CURRENT_SCHEMA_VERSION;
+        $headers['X-LaunchDarkly-Event-Schema'] = (string) EventPublisher::CURRENT_SCHEMA_VERSION;
         // Only the presence of this header is important. We encode a string
         // value of 'true' to ensure it isn't dropped along the way.
         $headers['X-LaunchDarkly-Unsummarized'] = 'true';
diff --git a/tests/Impl/UtilTest.php b/tests/Impl/UtilTest.php
index dd671246..640b0aa6 100644
--- a/tests/Impl/UtilTest.php
+++ b/tests/Impl/UtilTest.php
@@ -26,4 +26,16 @@ public function testNonTrivialSamplingRatio(): void
 
         $this->assertEquals(504, $counts);
     }
+
+    public function testEventHeaderValuesAreStrings(): void
+    {
+        // guzzlehttp/guzzle 7.11+ deprecates non-string header values and
+        // guzzle 8.0 will reject them, so every event header value must be a
+        // string. See https://github.com/launchdarkly/php-server-sdk/issues/246
+        $headers = Util::eventHeaders('sdk-key', []);
+
+        foreach ($headers as $name => $value) {
+            $this->assertIsString($value, "header '$name' should be a string");
+        }
+    }
 }