diff --git a/src/core/thread.c b/src/core/thread.c
index 06064bb2..b828f802 100644
--- a/src/core/thread.c
+++ b/src/core/thread.c
@@ -29,6 +29,7 @@
 
 #ifdef JANET_THREADS
 
+#include <math.h>
 #ifdef JANET_WINDOWS
 #include <windows.h>
 #else