diff --git a/src/filters/BasePSRFilter.php b/src/filters/BasePSRFilter.php
index <HASH>..<HASH> 100644
--- a/src/filters/BasePSRFilter.php
+++ b/src/filters/BasePSRFilter.php
@@ -52,7 +52,7 @@ abstract class BasePSRFilter implements Builder {
     $classes =
       (new Map($this->source->getAutoloadMap()['class']))->filterWithKey(
         function(string $class_name, string $file): bool {
-          if (\stripos($class_name, $this->prefix) !== 0) {
+          if ($this->prefix !== '' && \stripos($class_name, $this->prefix) !== 0) {
             return false;
           }
           $expected = static::getExpectedPathWithoutExtension(
Supporting PSR-{0,4} autoloading without a prefix

This is what Composer considers the "fallback" syntax.
<URL>