diff --git a/src/iksm.ts b/src/iksm.ts
index 87c6560..21a50b8 100644
--- a/src/iksm.ts
+++ b/src/iksm.ts
@@ -318,7 +318,7 @@ async function getSessionToken({
 }): Promise<string | undefined> {
   const fetch = wrapFetch({ cookieJar });
 
-  const res = await fetch(
+  const resp = await fetch(
     "https://accounts.nintendo.com/connect/1.0.0/api/session_token",
     {
       method: "POST",
@@ -338,8 +338,15 @@ async function getSessionToken({
       }),
     },
   );
-  const resBody = await res.json();
-  return resBody["session_token"];
+  const json = await resp.json();
+  if (json.error) {
+    throw new APIError({
+      response: resp,
+      json,
+      message: "Error getting session token",
+    });
+  }
+  return json["session_token"];
 }
 
 type IminkResponse = {
diff --git a/src/utils.ts b/src/utils.ts
index 0bbd6ed..d35e438 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -6,16 +6,16 @@ const stdinLines = io.readLines(Deno.stdin);
 
 export function urlBase64Encode(data: ArrayBuffer) {
   return base64.encode(data)
-    .replaceAll("+", "_")
-    .replaceAll("/", "-")
+    .replaceAll("+", "-")
+    .replaceAll("/", "_")
     .replaceAll("=", "");
 }
 
 export function urlBase64Decode(data: string) {
   return base64.decode(
     data
-      .replaceAll("_", "+")
-      .replaceAll("-", "/"),
+      .replaceAll("_", "/")
+      .replaceAll("-", "+"),
   );
 }