diff --git a/js-assembler/assemble-helpers.rkt b/js-assembler/assemble-helpers.rkt
index 5ed24e8..a501723 100644
--- a/js-assembler/assemble-helpers.rkt
+++ b/js-assembler/assemble-helpers.rkt
@@ -298,18 +298,24 @@
 
 (: assemble-label (Label Blockht -> String))
 (define (assemble-label a-label Blockht)
-  (let ([chunks
-         (regexp-split #rx"[^a-zA-Z0-9]+"
-                       (symbol->string (Label-name a-label)))])
+  (define a-block (hash-ref Blockht (Label-name a-label)))
+  (cond
+   [(block-looks-like-context-expected-values? a-block)
+    => (lambda (expected)
+         (format "RT.si_context_expected(~a)" expected))]
+   [else
+    (define chunks
+      (regexp-split #rx"[^a-zA-Z0-9]+"
+                    (symbol->string (Label-name a-label))))
     (cond
-      [(empty? chunks)
-       (error "impossible: empty label ~s" a-label)]
-      [(empty? (rest chunks))
-       (string-append "_" (first chunks))]
-      [else
-       (string-append "_"
-                      (first chunks)
-                      (apply string-append (map string-titlecase (rest chunks))))])))
+     [(empty? chunks)
+      (error "impossible: empty label ~s" a-label)]
+     [(empty? (rest chunks))
+      (string-append "_" (first chunks))]
+     [else
+      (string-append "_"
+                     (first chunks)
+                     (apply string-append (map string-titlecase (rest chunks))))])]))
 
 
 
diff --git a/js-assembler/assemble.rkt b/js-assembler/assemble.rkt
index 1d7f071..a96bc19 100644
--- a/js-assembler/assemble.rkt
+++ b/js-assembler/assemble.rkt
@@ -139,10 +139,23 @@ EOF
          [(symbol? stmt)
           (next)]
          [(LinkedLabel? stmt)
-          ;; Setting up multiple-value-return
-          (fprintf op "~a.mvr=~a;\n" 
-                   (assemble-label (make-Label (LinkedLabel-label stmt)) blockht)
-                   (assemble-label (make-Label (LinkedLabel-linked-to stmt)) blockht))
+          ;; Setting up multiple-value-return.
+          ;; Optimization: in the most common case (expecting only one), we optimize away
+          ;; the assignment, because there's a distinguished instruction, and it's implied
+          ;; that if .mvr is missing, that the block only expects one.
+          (define linked-to-block (hash-ref blockht (LinkedLabel-linked-to stmt)))
+          (cond
+           [(block-looks-like-context-expected-values? linked-to-block)
+            => (lambda (expected)
+                 (cond
+                  [(= expected 1)
+                   (void)]
+                  [else
+                   (fprintf op "~a.mvr=RT.si_context_expected(~a);\n" expected)]))]
+           [else
+            (fprintf op "~a.mvr=~a;\n" 
+                     (assemble-label (make-Label (LinkedLabel-label stmt)) blockht)
+                     (assemble-label (make-Label (LinkedLabel-linked-to stmt)) blockht))])
           (next)]
          [(DebugPrint? stmt)
           (next)]
diff --git a/js-assembler/runtime-src/baselib-functions.js b/js-assembler/runtime-src/baselib-functions.js
index 3cdb988..f77d278 100644
--- a/js-assembler/runtime-src/baselib-functions.js
+++ b/js-assembler/runtime-src/baselib-functions.js
@@ -72,12 +72,12 @@
             return MACHINE.c.pop().label(MACHINE);
         } else if (returnArgs.length === 0) {
             MACHINE.a = 0;
-            return MACHINE.c.pop().label.mvr(MACHINE);
+            return (MACHINE.c.pop().label.mvr || plt.runtime.si_context_expected_1)(MACHINE);
         } else {
             MACHINE.a = returnArgs.length;
             MACHINE.v = returnArgs.shift();
             MACHINE.e.push.apply(MACHINE.e, returnArgs.reverse());
-            return MACHINE.c.pop().label.mvr(MACHINE);
+            return (MACHINE.c.pop().label.mvr || plt.runtime.si_context_expected_1)(MACHINE);
         }
     };
 
diff --git a/js-assembler/runtime-src/runtime.js b/js-assembler/runtime-src/runtime.js
index 9cbce07..1a3e8e8 100644
--- a/js-assembler/runtime-src/runtime.js
+++ b/js-assembler/runtime-src/runtime.js
@@ -817,6 +817,7 @@
     exports['getTracedCalleeKey'] = getTracedCalleeKey;
 
     exports['si_context_expected'] = si_context_expected;
+    exports['si_context_expected_1'] = si_context_expected_1;
     exports['checkClosureAndArity'] = checkClosureAndArity;