diff --git a/a8/websocketsession.cc b/a8/websocketsession.cc
index c3aae61..e6081c1 100644
--- a/a8/websocketsession.cc
+++ b/a8/websocketsession.cc
@@ -212,16 +212,20 @@ namespace a8
             Close();
             return;
         }
+        if (opcode != BINARY_MODE) {
+            Close();
+            return;
+        }
         if (!is_final_frame) {
+            Close();
             return;
         }
 
         bool is_masked = (mask_payloadlen & 0x80) == 0x80;
-        if (opcode != BINARY_MODE) {
+        if (!is_masked) {
+            Close();
             return;
         }
-        assert(opcode == BINARY_MODE);
-        assert(is_masked);
 
         unsigned char payloadlen = mask_payloadlen & 0x7F;
         unsigned int framelen = 0;