diff --git a/onebusaway-sdk-kotlin-core/src/main/kotlin/org/onebusaway/core/ClientOptions.kt b/onebusaway-sdk-kotlin-core/src/main/kotlin/org/onebusaway/core/ClientOptions.kt
index 03d5540..1cccb10 100644
--- a/onebusaway-sdk-kotlin-core/src/main/kotlin/org/onebusaway/core/ClientOptions.kt
+++ b/onebusaway-sdk-kotlin-core/src/main/kotlin/org/onebusaway/core/ClientOptions.kt
@@ -172,7 +172,7 @@ private constructor(
             headers.put("X-Stainless-Runtime-Version", getJavaVersion())
             apiKey?.let {
                 if (!it.isEmpty()) {
-                    headers.put("key", it)
+                    queryParams.put("key", it)
                 }
             }
             headers.replaceAll(this.headers.build())