diff --git a/js/src/jsnum.cpp b/js/src/jsnum.cpp
--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -1569,17 +1569,17 @@ js::NonObjectToNumberSlow(ThreadSafeCont
         *out = v.toBoolean() ? 1.0 : 0.0;
         return true;
     }
     if (v.isNull()) {
         *out = 0.0;
         return true;
     }
 
-    JS_ASSERT(v.isUndefined());
+    JS_ASSERT(v.isUndefined() || v.isSymbol());
     *out = GenericNaN();
     return true;
 }
 
 #if defined(_MSC_VER)
 # pragma optimize("g", off)
 #endif
 
diff --git a/js/src/tests/ecma_6/Symbol/comparisons.js b/js/src/tests/ecma_6/Symbol/comparisons.js
new file mode 100644
--- /dev/null
+++ b/js/src/tests/ecma_6/Symbol/comparisons.js
@@ -0,0 +1,26 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/ */
+
+var symbols = [
+    Symbol(),
+    Symbol("0"),
+    Symbol.for("snowman"),
+    Symbol.iterator
+];
+
+var values = [
+    undefined, null, 0, 3.14, -0, NaN, "", "alphabet", Symbol("0"),
+    {}, []
+];
+
+for (var comparator of ["==", "!=", "===", "!==", "<", "<=", ">", ">="]) {
+    var f = Function("a, b", "return a " + comparator + " b;");
+    var expected = (comparator[0] == '!');
+    for (var a of symbols) {
+        for (var b of values)
+            assertEq(f(a, b), expected);
+    }
+}
+
+if (typeof reportCompare === "function")
+    reportCompare(0, 0);
diff --git a/js/src/tests/ecma_6/Symbol/conversions.js b/js/src/tests/ecma_6/Symbol/conversions.js
--- a/js/src/tests/ecma_6/Symbol/conversions.js
+++ b/js/src/tests/ecma_6/Symbol/conversions.js
@@ -8,11 +8,15 @@ var symbols = [
 ];
 
 for (var sym of symbols) {
     // 7.1.2 ToBoolean
     assertEq(Boolean(sym), true);
     assertEq(!sym, false);
     assertEq(sym || 13, sym);
     assertEq(sym && 13, 13);
+
+    // 7.1.3 ToNumber
+    assertEq(+sym, NaN);
+    assertEq(sym | 0, 0);
 }
 
 reportCompare(0, 0);