diff --git a/spec/tests.js b/spec/tests.js
index 1537f1d..180a0b3 100644
--- a/spec/tests.js
+++ b/spec/tests.js
@@ -225,7 +225,19 @@ module.exports = [
         value1: {toString: ()=>'Hello world!'},
         value2: {toString: ()=>'Hi!'},
         equal: false
-      }
+      },
+      {
+        description: "objects with `valueOf` property that isn't a function don't throw",
+        value1: {valueOf: {}},
+        value2: {valueOf: {}},
+        equal: true
+      },
+      {
+        description: "objects with `toString` property that isn't a function don't throw",
+        value1: {toString: {}},
+        value2: {toString: {}},
+        equal: true
+      },
     ]
   },
 
diff --git a/src/index.jst b/src/index.jst
index 5d4ee2f..be8614d 100644
--- a/src/index.jst
+++ b/src/index.jst
@@ -48,8 +48,12 @@ module.exports = function equal(a, b) {
 {{?}}
 
     if (a.constructor === RegExp) return a.source === b.source && a.flags === b.flags;
-    if (a.valueOf !== Object.prototype.valueOf) return a.valueOf() === b.valueOf();
-    if (a.toString !== Object.prototype.toString) return a.toString() === b.toString();
+    if ((a.valueOf !== Object.prototype.valueOf || b.valueOf !== Object.prototype.valueOf)
+        && typeof a.valueOf === 'function' && typeof b.valueOf === 'function')
+      return a.valueOf() === b.valueOf();
+    if ((a.toString !== Object.prototype.toString || b.toString !== Object.prototype.toString)
+        && typeof a.toString === 'function' && typeof b.toString === 'function')
+      return a.toString() === b.toString();
 
     keys = Object.keys(a);
     length = keys.length;