diff --git a/index.js b/index.js
index 260beb8..89e51f2 100644
--- a/index.js
+++ b/index.js
@@ -16,6 +16,12 @@ function isReadable(stream) {
   return true;
 }
 
+function assertReadableStream(stream) {
+  if (!isReadable(stream)) {
+    throw new Error('All input streams must be readable');
+  }
+}
+
 function OrderedStreams(streams, options) {
   streams = streams || [];
 
@@ -25,6 +31,8 @@ function OrderedStreams(streams, options) {
 
   streams = Array.prototype.concat.apply([], streams);
 
+  streams.forEach(assertReadableStream);
+
   options = Object.assign({}, options, {
     read: read,
     predestroy: predestroy,
@@ -43,11 +51,9 @@ function OrderedStreams(streams, options) {
   var destroyedByError = false;
   var readableClosed = false;
 
-  streams.forEach(function (stream, idx) {
-    if (!isReadable(stream)) {
-      throw new Error('All input streams must be readable');
-    }
+  streams.forEach(setup);
 
+  function setup(stream, idx) {
     stream.on('data', onData);
     stream.once('error', onError);
     stream.once('end', onEnd);
@@ -85,23 +91,25 @@ function OrderedStreams(streams, options) {
       cleanup();
       readable.destroy();
     }
-  });
+  }
 
   function predestroy() {
-    streams.forEach(function (stream, idx) {
-      if (destroyedIdx === idx) {
-        return;
-      }
+    streams.forEach(destroyStream);
+  }
 
-      if (destroyedByError) {
-        return stream.destroy();
-      }
-      if (readableClosed) {
-        return stream.destroy();
-      }
+  function destroyStream(stream, idx) {
+    if (destroyedIdx === idx) {
+      return;
+    }
+
+    if (destroyedByError) {
+      return stream.destroy();
+    }
+    if (readableClosed) {
+      return stream.destroy();
+    }
 
-      stream.destroy(new Error('Wrapper destroyed'));
-    });
+    stream.destroy(new Error('Wrapper destroyed'));
   }
 
   function onData(chunk) {