diff --git a/include/math.lusi b/include/math.lusi
index 68b2d3826365e90916b5fb7852eac320f375de66..e320e6e48511c83683a16f497fbfa61f63e7c084 100644
--- a/include/math.lusi
+++ b/include/math.lusi
@@ -11,6 +11,7 @@ function cosh (x: real) returns (y: real) prototype C lib m;
 function ceil (x: real) returns (y: real) prototype C lib m;
 function erf (x: real) returns (y: real) prototype C lib m;
 function fabs (x: real) returns (y: real) prototype C lib m;
+function tan (x: real) returns (y: real) prototype C lib m;
 function pow (x:real; n: real) returns (y: real) prototype C lib m;
 function sin (x: real) returns (y: real) prototype C lib m;
 function sinh (x: real) returns (y: real) prototype C lib m;