Index: generic/nsfObj.c
===================================================================
diff -u -r658c5ca127345b3f4377e551d5e9517c6481b473 -r207a8f514cdf626259fdbfa3bc525bdb0339efa0
--- generic/nsfObj.c	(.../nsfObj.c)	(revision 658c5ca127345b3f4377e551d5e9517c6481b473)
+++ generic/nsfObj.c	(.../nsfObj.c)	(revision 207a8f514cdf626259fdbfa3bc525bdb0339efa0)
@@ -423,9 +423,10 @@
   NsfClass *mixin = NULL;
   Tcl_Obj *guardObj = NULL, *nameObj = NULL;
   Mixinreg *mixinRegPtr;
-  int oc; Tcl_Obj **ov;
+  int oc, result; Tcl_Obj **ov;
 
-  if (Tcl_ListObjGetElements(interp, objPtr, &oc, &ov) == TCL_OK) {
+  result = Tcl_ListObjGetElements(interp, objPtr, &oc, &ov);
+  if (result == TCL_OK) {
 
     if (oc == 1) {
       nameObj = ov[0];
@@ -443,7 +444,8 @@
     }
 
   } else {
-    return NsfObjErrType(interp, "mixin", nameObj, "a class as mixin", NULL);
+    /* invalid Tcl list */
+    return result;
   }
 
   /*
Index: tests/interceptor-slot.test
===================================================================
diff -u -r0f881e4bc45e927c8d84c1b1b468ef7537cb9b03 -r207a8f514cdf626259fdbfa3bc525bdb0339efa0
--- tests/interceptor-slot.test	(.../interceptor-slot.test)	(revision 0f881e4bc45e927c8d84c1b1b468ef7537cb9b03)
+++ tests/interceptor-slot.test	(.../interceptor-slot.test)	(revision 207a8f514cdf626259fdbfa3bc525bdb0339efa0)
@@ -158,6 +158,31 @@
   ? {O info precedence} "::M1 ::nx::Class ::nx::Object"
 }
 
+nx::test case mixin-relation-invalid-value {
+  
+  nx::Class create C
+  nx::Class create M
+  
+  # Double-escaping allows for injecting an invalid string rep of a Tcl
+  # list into MixinregSetFromAny(). Other API facets (::nsf::mixin)
+  # catch this through list ops at the script level.
+  
+  set ::str "\\\{ ::M"
+  ? {lindex [subst $::str] 0} "unmatched open brace in list"
+  ? {C mixins get} ""
+  ? {C object mixins get} ""
+  ? {::nsf::relation::set C class-mixin $::str} "unmatched open brace in list"
+  ? {::nsf::relation::set C object-mixin $::str} "unmatched open brace in list"
+  ? {C mixins get} ""
+  ? {C object mixins get} ""
+
+  # via slot interface
+  ? {C mixins set $::str} "unmatched open brace in list"
+  ? {C object mixins set $::str} "unmatched open brace in list"
+  
+}
+
+
 nx::test case filter-relation {
   nx::Class create CC {
     :public method filterA args {next}