diff --git a/core/compiler/test/stack-trace.factor b/core/compiler/test/stack-trace.factor
index ee94f0c9a2..4c47ca8a12 100644
--- a/core/compiler/test/stack-trace.factor
+++ b/core/compiler/test/stack-trace.factor
@@ -1,25 +1,28 @@
 IN: temporary
 USING: compiler tools.test namespaces sequences
 kernel.private kernel math continuations continuations.private
-words ;
+words splitting ;
 
 : symbolic-stack-trace ( -- newseq )
-    error-continuation get continuation-call callstack>array ;
+    error-continuation get continuation-call callstack>array
+    2 group flip first ;
 
 : foo 3 throw 7 ;
 : bar foo 4 ;
 : baz bar 5 ;
 \ baz compile
 [ 3 ] [ [ baz ] catch ] unit-test
-[ { baz bar foo throw } ] [
-    symbolic-stack-trace [ word? ] subset
+[ t ] [
+    symbolic-stack-trace
+    [ word? ] subset
+    { baz bar foo throw } tail?
 ] unit-test
 
 : bleh [ 3 + ] map [ 0 > ] subset ;
 \ bleh compile
 
 : stack-trace-contains? symbolic-stack-trace memq? ;
-    
+
 [ t ] [
     [ { 1 "hi" } bleh ] catch drop \ + stack-trace-contains?
 ] unit-test
diff --git a/core/generator/registers/registers.factor b/core/generator/registers/registers.factor
index 9c860603eb..7e31f83abc 100644
--- a/core/generator/registers/registers.factor
+++ b/core/generator/registers/registers.factor
@@ -365,17 +365,17 @@ M: object minimal-ds-loc* drop ;
     2dup [ length ] 2apply <=
     [ drop { } swap ] [ length swap cut* ] if ;
 
-: substitute-vregs ( alist -- )
-    >hashtable
-    { phantom-d phantom-r }
-    [ get substitute ] curry* each ;
+: vreg-substitution ( value vreg -- pair )
+    dupd <cached> 2array ;
+
+: substitute-vregs ( values vregs -- )
+    [ vreg-substitution ] 2map [ first loc? ] subset >hashtable
+    [ swap substitute ] curry each-phantom ;
 
 : lazy-load ( values template -- )
     #! Set operand vars here.
-    flip first2
-    >r dupd [ (lazy-load) ] 2map dup r>
-    [ >r dup value? [ value-literal ] when r> set ] 2each
-    dupd [ <cached> ] 2map 2array flip [ first loc? ] subset
+    2dup [ first (lazy-load) ] 2map dup rot
+    [ >r dup value? [ value-literal ] when r> second set ] 2each
     substitute-vregs ;
 
 : fast-input ( template -- )