diff --git a/op.c b/op.c
index ff2848a29016..a08be2efc0f1 100644
--- a/op.c
+++ b/op.c
@@ -12314,6 +12314,15 @@ S_aassign_scan(pTHX_ OP* o, bool rhs, bool top, int *scalars_p)
         break;
 
     case OP_UNDEF:
+        /* undef counts as a scalar on the RHS:
+         *   (undef, $x) = ...;         # only 1 scalar on LHS: always safe
+         *   ($x, $y)    = (undef, $x); # 2 scalars on RHS: unsafe
+         */
+        if (rhs)
+            (*scalars_p)++;
+        flags = AAS_SAFE_SCALAR;
+        break;
+
     case OP_PUSHMARK:
     case OP_STUB:
         /* these are all no-ops; they don't push a potentially common SV
@@ -14247,7 +14256,7 @@ Perl_rpeep(pTHX_ OP *o)
                 || !r                      /* .... = (); */
                 || !(l & ~AAS_SAFE_SCALAR) /* (undef, pos()) = ...; */
                 || !(r & ~AAS_SAFE_SCALAR) /* ... = (1,2,length,undef); */
-                || (lscalars < 2)          /* ($x) = ... */
+                || (lscalars < 2)          /* ($x, undef) = ... */
             ) {
                 NOOP; /* always safe */
             }
@@ -14291,7 +14300,7 @@ Perl_rpeep(pTHX_ OP *o)
 
             /* ... = ($x)
              * may have to handle aggregate on LHS, but we can't
-             * have common scalars*/
+             * have common scalars. */
             if (rscalars < 2)
                 o->op_private &=
                         ~(OPpASSIGN_COMMON_SCALAR|OPpASSIGN_COMMON_RC1);
diff --git a/pp_hot.c b/pp_hot.c
index dd991ae7313f..bed0a27ce369 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -1102,6 +1102,7 @@ S_aassign_copy_common(pTHX_ SV **firstlelem, SV **lastlelem,
 
 #ifdef DEBUGGING
             if (fake) {
+                /* op_dump(PL_op); */
                 Perl_croak(aTHX_
                     "panic: aassign skipped needed copy of common RH elem %"
                         UVuf, (UV)(relem - firstrelem));
diff --git a/t/op/aassign.t b/t/op/aassign.t
index 58650b72f369..0fe74c95e665 100644
--- a/t/op/aassign.t
+++ b/t/op/aassign.t
@@ -334,5 +334,13 @@ SKIP: {
 
 }
 
+{
+    my $x = 1;
+    my $y = 2;
+    ($x,$y) = (undef, $x);
+    is($x, undef, 'single scalar on RHS, but two on LHS: x');
+    is($y, 1, 'single scalar on RHS, but two on LHS: y');
+}
+
 
 done_testing();
diff --git a/t/perf/benchmarks b/t/perf/benchmarks
index 6baa3b29dad1..7fcc1fd253ef 100644
--- a/t/perf/benchmarks
+++ b/t/perf/benchmarks
@@ -618,6 +618,11 @@
     'expr::aassign::2l_1l' => {
         desc    => 'single lexical RHS',
         setup   => 'my $x = 1;',
+        code    => '($x,$x) = ($x)',
+    },
+    'expr::aassign::2l_1ul' => {
+        desc    => 'undef and single lexical RHS',
+        setup   => 'my $x = 1;',
         code    => '($x,$x) = (undef, $x)',
     },