diff --git a/quiche/spdy/core/spdy_protocol.h b/quiche/spdy/core/spdy_protocol.h
index 7f462a5..2368f9c 100644
--- a/quiche/spdy/core/spdy_protocol.h
+++ b/quiche/spdy/core/spdy_protocol.h
@@ -279,7 +279,7 @@
 
 // Serialize RST_STREAM or GOAWAY frame error code to string
 // for logging/debugging.
-const char* ErrorCodeToString(SpdyErrorCode error_code);
+QUICHE_EXPORT_PRIVATE const char* ErrorCodeToString(SpdyErrorCode error_code);
 
 // Serialize |type| to string for logging/debugging.
 QUICHE_EXPORT_PRIVATE const char* WriteSchedulerTypeToString(