diff --git a/src/libiso8601/400_calc.h b/src/libiso8601/400_calc.h
index 9570d8d..bf15b8d 100644
--- a/src/libiso8601/400_calc.h
+++ b/src/libiso8601/400_calc.h
@@ -26,11 +26,7 @@ understood by humans, and vice versa.
 Returns non-0 if \a year is a leap year.
 
 */
-int iso8601_isleap(int year)
-#ifndef DOXYGEN
-    __attribute__((nonnull))
-#endif
-;
+int iso8601_isleap(int year);