diff --git a/SocketRocket/SRWebSocket.m b/SocketRocket/SRWebSocket.m
index 4e30aef54..02776760a 100644
--- a/SocketRocket/SRWebSocket.m
+++ b/SocketRocket/SRWebSocket.m
@@ -1015,7 +1015,7 @@ - (void)_readFrameContinue;
                 }
 
                 if (header.masked) {
-                    assert(mapped_size >= sizeof(_currentReadMaskOffset) + offset);
+                    assert(mapped_size >= sizeof(_currentReadMaskKey) + offset);
                     memcpy(eself->_currentReadMaskKey, ((uint8_t *)mapped_buffer) + offset, sizeof(eself->_currentReadMaskKey));
                 }