diff --git a/lib/sandbox.js b/lib/sandbox.js
index 6a59d29..8437e11 100644
--- a/lib/sandbox.js
+++ b/lib/sandbox.js
@@ -53,11 +53,12 @@ function runExtension(sessionContext, key, extCall, callback) {
   else {
     re[key] = [callback];
     extensions.run(extCall, function(err, fault, res) {
+      const callbacks = re[key];
+      delete re[key];
       if (!err && !fault)
         sessionContext.extensionsCache[key] = res;
-      for (let c of re[key])
+      for (let c of callbacks)
         c(err, fault);
-      delete re[key];
     });
   }
 }