diff --git a/common.scad b/common.scad
index 551afa4..e6945fd 100644
--- a/common.scad
+++ b/common.scad
@@ -85,14 +85,14 @@ module screw_hole_lobe(ID, OD, lobes=3) {
 
 module circle_outer(d) {
 	// Regular circle inscribes, this circumscribes
-	$fn = is_undef($fn) ? 72 : $fn;
+	$fn = (is_undef($fn) || $fn<3) ? 72 : $fn;
 	fudge = 1/cos(180/$fn);
 	circle(d=d*fudge);
 }
 
 module cylinder_outer(d, h, center=false) {
 	// Regular circle inscribes, this circumscribes
-	$fn = is_undef($fn) ? 72 : $fn;
+	$fn = (is_undef($fn) || $fn<3) ? 72 : $fn;
 	fudge = 1/cos(180/$fn);
 	cylinder(d=d*fudge, h=h, center=center);
 }