diff --git a/index.js b/index.js
index f34f074..64a72f8 100644
--- a/index.js
+++ b/index.js
@@ -78,7 +78,7 @@ function replaceString(str, match, fn) {
 
     curCharLen = result[i].length;
     curCharStart += result[i - 1].length;
-    result[i] = fn(result[i], i, curCharStart);
+    result[i] = fn(result[i], (i - 1) / 2, curCharStart);
     curCharStart += curCharLen;
   }
 
diff --git a/test.js b/test.js
index 761d43d..ab22e88 100644
--- a/test.js
+++ b/test.js
@@ -170,3 +170,14 @@ test("Avoids undefined values due to regex", (t) => {
     replaceString(string, /(hey)|(you)/, x => x);
   });
 });
+
+test("Indexes start at 0 and are contiguous", t => {
+  const string = 'Hello there general Kenobi';
+  const re = /(\w+)/;
+
+  let expectedIndex = 0;
+  replaceString(string, re, (match, index) => {
+    t.deepEqual(expectedIndex, index);
+    expectedIndex++;
+  });
+});