diff --git a/common.scad b/common.scad
index dc37354..505a2cc 100644
--- a/common.scad
+++ b/common.scad
@@ -74,12 +74,16 @@ function all_defined(v,recursive=false) = max([for (x=v) is_undef(x)||(recursive
 
 // Function: get_radius()
 // Usage:
-//   get_radius([r1], [r], [d1], [d], [dflt]);
+//   get_radius([r1], [r2], [r], [d1], [d2], [d], [dflt]);
 // Description:
 //   Given various radii and diameters, returns the most specific radius.
 //   If a diameter is most specific, returns half its value, giving the radius.
 //   If no radii or diameters are defined, returns the value of dflt.
-//   Value specificity order is r1, d1, r, d, then dflt
+//   Value specificity order is r1, r2, d1, d2, r, d, then dflt
+//   Only one of `r1`, `r2`, `d1`, or `d2` can be defined at once, or else it
+//   errors out, complaining about conflicting radius/diameter values.
+//   Only one of `r` or `d` can be defined at once, or else it errors out,
+//   complaining about conflicting radius/diameter values.
 // Arguments:
 //   r1 = Most specific radius.
 //   d1 = Most specific diameter.
@@ -89,11 +93,11 @@ function all_defined(v,recursive=false) = max([for (x=v) is_undef(x)||(recursive
 //   d = Most general diameter.
 //   dflt = Value to return if all other values given are `undef`.
 function get_radius(r1=undef, r2=undef, r=undef, d1=undef, d2=undef, d=undef, dflt=undef) = (
-	!is_undef(r1)? r1 :
-	!is_undef(r2)? r2 :
+	!is_undef(r1)? assert(is_undef(r2)&&is_undef(d1)&&is_undef(d2), "Conflicting or redundant radius/diameter arguments given.") r1 :
+	!is_undef(r2)? assert(is_undef(d1)&&is_undef(d2), "Conflicting or redundant radius/diameter arguments given.") r2 :
 	!is_undef(d1)? d1/2 :
 	!is_undef(d2)? d2/2 :
-	!is_undef(r)? r :
+	!is_undef(r)? assert(is_undef(d), "Conflicting or redundant radius/diameter arguments given.") r :
 	!is_undef(d)? d/2 :
 	dflt
 );