diff --git a/venn.js b/venn.js
index 89f840c..5de0f1b 100644
--- a/venn.js
+++ b/venn.js
@@ -238,13 +238,13 @@
             fB = f(b),
             delta = b - a;
 
+        if (Math.abs(fA) < tolerance) return a;
+        if (Math.abs(fB) < tolerance) return b;
+
         if (fA * fB > 0) {
             throw "Initial bisect points must have opposite signs";
         }
 
-        if (fA === 0) return a;
-        if (fB === 0) return b;
-
         for (var i = 0; i < maxIterations; ++i) {
             delta /= 2;
             var mid = a + delta,