diff --git a/build.sh b/build.sh
index 686fd65..56d4728 100755
--- a/build.sh
+++ b/build.sh
@@ -31,8 +31,10 @@ guess_sdk_version()
         guess_sdk_version_result=$tmp3
         echo 'found SDK version' $SDK_VERSION 'at tarballs/'$sdk
     fi
-    if [ $SDK_VERSION -eq 10.4 ]; then
-        SDK_VERSION=10.4u
+    if [ $guess_sdk_version_result ]; then
+        if [ $guess_sdk_version_result -eq 10.4 ]; then
+            guess_sdk_version_result=10.4u
+        fi
     fi
     export guess_sdk_version_result
 }