diff --git a/shapes3d.scad b/shapes3d.scad
index 96c570a..a8c3db0 100644
--- a/shapes3d.scad
+++ b/shapes3d.scad
@@ -1012,20 +1012,21 @@ function regular_prism(n,
         tex_depth = default(tex_depth,1),
         height = one_defined([l, h, length, height],"l,h,length,height",dflt=1),
         sc = 1/cos(180/n),
-        ir = u_mul(ir,sc),
-        ir1 = u_mul(first_defined([ir1,ir,r]), sc),
-        ir2 = u_mul(first_defined([ir2,ir,r]), sc),
-        id1 = u_mul(first_defined([id1,id,d]), sc),
-        id2 = u_mul(first_defined([id2,id,d]), sc),
+        ir1 = u_mul(default(ir1,ir), sc),
+        ir2 = u_mul(default(ir2,ir), sc),
+        id1 = u_mul(default(id1,id), sc),
+        id2 = u_mul(default(id2,id), sc),
         od1 = default(od1,od),
         od2 = default(od2,od),
         or1 = default(or1,or),
-        or2 = default(or2,or), 
+        or2 = default(or2,or),
+        d1 = default(d1,d),
+        d2 = default(d2,d),
         side = is_finite(side)? side/2/sin(180/n) : undef,
         side1 = is_finite(side1)? side1/2/sin(180/n) : side,
         side2 = is_finite(side2)? side2/2/sin(180/n) : side,
-        r1 = get_radius(r1=ir1,r2=or1,r=r1,d=d1,d1=id1,d2=od1,dflt=side1),
-        r2 = get_radius(r1=ir2,r2=or2,r=r2,d=d2,d1=id2,d2=od2,dflt=side2),
+        r1 = get_radius(r1=ir1,r2=or1,r=default(r1,r),d=d1,d1=id1,d2=od1,dflt=side1),
+        r2 = get_radius(r1=ir2,r2=or2,r=default(r2,r),d=d2,d1=id2,d2=od2,dflt=side2),
         anchor = get_anchor(anchor,center,BOT,CENTER)
     )
     assert(num_defined([side,od,id,or,ir])<=1, "Can only define one of side, id, od, ir, and or")