diff --git a/arch/x64/math.cc b/arch/x64/math.cc
index 0697b247e0e80499985d5ee2d47d75a9d7eeccd2..57cba9118b0331003d49dfc5af575bb4c7b90175 100644
--- a/arch/x64/math.cc
+++ b/arch/x64/math.cc
@@ -17,6 +17,7 @@ double floor(double v)
     return r;
 }
 
+extern "C"
 int __isnan(double v)
 {
     u64 r;