diff --git a/autotests/urlutiltest.cpp b/autotests/urlutiltest.cpp
--- a/autotests/urlutiltest.cpp
+++ b/autotests/urlutiltest.cpp
@@ -37,11 +37,21 @@
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/data/documents/muh"), lUrl("/home/test")), lUrl("/home/test/data"));
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/data/documents/muh/"), lUrl("/home/test")), lUrl("/home/test/data"));
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/data/documents/muh"), lUrl("/home/test/")), lUrl("/home/test/data"));
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/a/"), lUrl("/home")), lUrl("/home/a"));
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/te"), lUrl("/")), lUrl("/te"));
+    // One letter under root is also a valid child
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/d"), lUrl("/")), lUrl("/d"));
+    // Same urls sould return QUrl()
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/data"), lUrl("/home/test/data/")), QUrl());
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/data/"), lUrl("/home/test/data")), QUrl());
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/test/"), lUrl("/home/test/")), QUrl());
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/"), lUrl("/")), QUrl());
+    // Not related urls should return QUrl()
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/aaa/"), lUrl("/home/bbb/")), QUrl());
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/aaa/"), lUrl("/home/bbb/ccc")), QUrl());
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home"), lUrl("/test")), QUrl());
+    // Child urls in reverse order should return QUrl()
+    QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home"), lUrl("/home/test")), QUrl());
     // # is %23 in an URL, so this test could reveal path/URL confusion in the code:
     QCOMPARE(KIO::UrlUtil::firstChildUrl(lUrl("/home/a#/b#"), lUrl("/home/a#")), lUrl("/home/a#/b#"));
 }
diff --git a/src/filewidgets/urlutil_p.h b/src/filewidgets/urlutil_p.h
--- a/src/filewidgets/urlutil_p.h
+++ b/src/filewidgets/urlutil_p.h
@@ -66,13 +66,21 @@
 
         const QString childPath = adjustedLastUrl.path();
         const QString parentPath = adjustedCurrentUrl.path();
+        // if the parent path is root "/"
+        // one char more is a valid path, otherwise "/" and another char are needed.
+        const int minIndex = (parentPath == QLatin1String("/")) ? 1 : 2;
 
         // e.g. this would just be ok:
+        // childPath  = /a        len=2
+        // parentPath = /         len=1
         // childPath  = /home/a   len=7
         // parentPath = /home     len=5
-        Q_ASSERT(childPath.length() >= (parentPath.length() + 2));
 
-        const int idx2 = childPath.indexOf(QLatin1Char('/'), parentPath.length() + 2);
+        if (childPath.length() < (parentPath.length() + minIndex) ) {
+            return QUrl();
+        }
+
+        const int idx2 = childPath.indexOf(QLatin1Char('/'), parentPath.length() + minIndex);
         // parentPath = /home
         // childPath  = /home/a
         //                 idx  = -1