diff --git a/proton b/proton
index d9b37d15..fbb5f879 100755
--- a/proton
+++ b/proton
@@ -34,6 +34,10 @@ def upgrade_pfx(old_ver):
         return
 
     log("Upgrading prefix from " + str(old_ver) + " to " + CURRENT_PREFIX_VERSION + " (" + os.environ["STEAM_COMPAT_DATA_PATH"] + ")")
+
+    if old_ver == None:
+        return
+
     old_proton_ver, old_prefix_ver = old_ver.split('-')
 
     if old_proton_ver == "3.7" and old_prefix_ver == "1":