diff --git a/src/c/hash.d b/src/c/hash.d
index 69d04ec72..0780521f2 100644
--- a/src/c/hash.d
+++ b/src/c/hash.d
@@ -15,6 +15,8 @@
     See file '../Copyright' for full details.
 */
 
+/* for ECL_MATHERR_* */
+#define ECL_INCLUDE_MATH_H
 #include <ecl/ecl.h>
 #include <stdlib.h>
 #include <string.h>