diff --git a/src/gen/gendaymtx.c b/src/gen/gendaymtx.c
index 08f05d255..507072068 100644
--- a/src/gen/gendaymtx.c
+++ b/src/gen/gendaymtx.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: gendaymtx.c,v 2.20 2015/08/01 23:27:04 greg Exp $";
+static const char RCSid[] = "$Id: gendaymtx.c,v 2.21 2015/09/02 22:52:04 greg Exp $";
 #endif
 /*
  *  gendaymtx.c
@@ -395,6 +395,11 @@ main(int argc, char *argv[])
 		case '5':			/* 5-phase calculation */
 			nsuns = 1;
 			fixed_sun_sa = PI/360.*atof(argv[++i]);
+			if (fixed_sun_sa <= 0) {
+				fprintf(stderr, "%s: missing solar disk size argument for '-5' option\n",
+						argv[0]);
+				exit(1);
+			}
 			fixed_sun_sa *= fixed_sun_sa*PI;
 			break;
 		default: