diff --git a/NEWS b/NEWS
index bb541bba3..83388e38a 100644
--- a/NEWS
+++ b/NEWS
@@ -1,4 +1,5 @@
 ver 0.21.19 (not yet released)
+* fix build failure with clang 10
 
 ver 0.21.18 (2019/12/24)
 * protocol
diff --git a/src/util/Compiler.h b/src/util/Compiler.h
index 96f63fae4..04e49bb61 100644
--- a/src/util/Compiler.h
+++ b/src/util/Compiler.h
@@ -145,7 +145,7 @@
 
 #if GCC_CHECK_VERSION(7,0)
 #define gcc_fallthrough __attribute__((fallthrough))
-#elif CLANG_CHECK_VERSION(10,0)
+#elif CLANG_CHECK_VERSION(10,0) && defined(__cplusplus)
 #define gcc_fallthrough [[fallthrough]]
 #else
 #define gcc_fallthrough