diff --git a/src/objective-c/tests/InteropTests.m b/src/objective-c/tests/InteropTests.m
index 766fa19d85718afff97c09e44eb58ca66b45e52d..d964f53e8e2c0b6d5cb6ea7495b6d279aa13f934 100644
--- a/src/objective-c/tests/InteropTests.m
+++ b/src/objective-c/tests/InteropTests.m
@@ -90,6 +90,9 @@
   return nil;
 }
 
+// This number indicates how many bytes of overhead does Protocol Buffers encoding add onto the
+// message. The number varies as different message.proto is used on different servers. The actual
+// number for each interop server is overridden in corresponding derived test classes.
 - (int32_t)encodingOverhead {
   return 0;
 }