diff --git a/src/lisp/fdlibm.h b/src/lisp/fdlibm.h
index 61794b3f79eb32590304e22cee8cc76398b9419c..467daa5379a01e9cc615e220dc122d5317a71a72 100644
--- a/src/lisp/fdlibm.h
+++ b/src/lisp/fdlibm.h
@@ -10,6 +10,8 @@
  * ====================================================
  */
 
+#ifndef FDLIBM_H_
+#define FDLIBM_H_
 /* Sometimes it's necessary to define __LITTLE_ENDIAN explicitly
    but these catch some common cases. */
 
@@ -48,3 +50,5 @@ extern double __kernel_tan(double x, double y, int iy);
 extern double sin(double x);
 extern double cos(double x);
 extern double tan(double x);
+
+#endif