diff --git a/src/FunctionsClient.ts b/src/FunctionsClient.ts
index ed1849f..a84cb08 100644
--- a/src/FunctionsClient.ts
+++ b/src/FunctionsClient.ts
@@ -86,6 +86,9 @@ export class FunctionsClient {
           _headers['Content-Type'] = 'application/json'
           body = JSON.stringify(functionArgs)
         }
+      } else {
+        // if the Content-Type was supplied, simply set the body
+        body = functionArgs;
       }
 
       const response = await this.fetch(`${this.url}/${functionName}`, {