diff --git a/src/h/external.h b/src/h/external.h
index df810c718..5956b9034 100755
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -1118,7 +1118,7 @@ extern ECL_API long double ecl_to_long_double(cl_object x);
 extern ECL_API cl_object ecl_make_long_float(long double f);
 #endif
 #ifdef ECL_IEEE_FP
-extern cl_object si_nan();
+extern ECL_API cl_object si_nan();
 #endif /* ECL_IEEE_FP */
 
 /* num_co.c */