diff --git a/include/c99_math.h b/include/c99_math.h
index bd35d1b32ed..5b01d53a83a 100644
--- a/include/c99_math.h
+++ b/include/c99_math.h
@@ -71,6 +71,7 @@ roundf(float x)
 #endif
 
 #ifndef INFINITY
+#include <float.h> // DBL_MAX
 #define INFINITY (DBL_MAX + DBL_MAX)
 #endif