diff --git a/index.js b/index.js
index ac8aaf2..ab850d3 100644
--- a/index.js
+++ b/index.js
@@ -18,12 +18,17 @@ function findAllAfter(parent, index, test) {
   children = parent.children
   length = children.length
 
-  if (index && index.type) {
+  if (index === undefined || index === null) {
+    throw new Error('Expected positive finite index or child node')
+  } else if (index && typeof index !== 'number') {
     index = children.indexOf(index)
+    if (index === -1) {
+      throw new Error('Expected child node')
+    }
   }
 
-  if (isNaN(index) || index < 0 || index === Infinity) {
-    throw new Error('Expected positive finite index or child node')
+  if (typeof index !== 'number' || index < 0 || index === Infinity) {
+    throw new Error('Expected positive finite number as index')
   }
 
   while (++index < length) {
diff --git a/test.js b/test.js
index 5a31a35..516c9da 100644
--- a/test.js
+++ b/test.js
@@ -30,15 +30,29 @@ test('unist-util-find-all-after', function(t) {
     assert.throws(function() {
       findAllAfter({type: 'foo', children: []})
     }, /Expected positive finite index or child node/)
+  }, 'should fail without index')
 
+  t.doesNotThrow(function() {
     assert.throws(function() {
       findAllAfter({type: 'foo', children: []}, -1)
-    }, /Expected positive finite index or child node/)
+    }, /Expected positive finite number as index/)
+
+    assert.throws(function() {
+      findAllAfter({type: 'foo', children: []}, Infinity)
+    }, /Expected positive finite number as index/)
+
+    assert.throws(function() {
+      findAllAfter({type: 'foo', children: []}, false)
+    }, /Expected positive finite number as index/)
+
+    assert.throws(function() {
+      findAllAfter({type: 'foo', children: []}, '')
+    }, /Expected positive finite number as index/)
 
     assert.throws(function() {
       findAllAfter({type: 'foo', children: []}, {type: 'bar'})
-    }, /Expected positive finite index or child node/)
-  }, 'should fail without index')
+    }, /Expected child node/)
+  }, 'should fail with invalid index')
 
   t.doesNotThrow(function() {
     assert.throws(function() {