diff --git a/core/fdrm/fx_crypt.cpp b/core/fdrm/fx_crypt.cpp
index 7e0ce7f..3d388d7 100644
--- a/core/fdrm/fx_crypt.cpp
+++ b/core/fdrm/fx_crypt.cpp
@@ -147,7 +147,8 @@
 
   int j = 0;
   for (int i = 0; i < kRC4ContextPermutationLength; ++i) {
-    j = (j + context->m[i] + (key.size() ? key[i % key.size()] : 0)) & 0xFF;
+    size_t size = key.size();
+    j = (j + context->m[i] + (size ? key[i % size] : 0)) & 0xFF;
     std::swap(context->m[i], context->m[j]);
   }
 }