diff --git a/autotests/kurlcompletiontest.cpp b/autotests/kurlcompletiontest.cpp
--- a/autotests/kurlcompletiontest.cpp
+++ b/autotests/kurlcompletiontest.cpp
@@ -51,6 +51,7 @@
     void testLocalURL();
     void testEmptyCwd();
     void testBug346920();
+    void testInvalidProtocol();
     void testUser();
     void testCancel();
 
@@ -339,7 +340,6 @@
 
 void KUrlCompletionTest::testEmptyCwd()
 {
-    qDebug();
     // Completion with empty string (with a KUrlCompletion whose cwd is "")
     qDebug() << endl << "now completing on '' with empty cwd";
     m_completionEmptyCwd->makeCompletion(QLatin1String(""));
@@ -356,6 +356,14 @@
     // just don't crash
 }
 
+void KUrlCompletionTest::testInvalidProtocol()
+{
+    m_completion->makeCompletion(QStringLiteral(":/"));
+    waitForCompletion(m_completion);
+    m_completion->allMatches();
+    // just don't crash
+}
+
 void KUrlCompletionTest::testUser()
 {
     m_completionEmptyCwd->makeCompletion(QStringLiteral("~"));
@@ -427,6 +435,7 @@
     testLocalURL();
     testEmptyCwd();
     testBug346920();
+    testInvalidProtocol();
     testUser();
     testCancel();
     teardown();
diff --git a/src/widgets/kurlcompletion.cpp b/src/widgets/kurlcompletion.cpp
--- a/src/widgets/kurlcompletion.cpp
+++ b/src/widgets/kurlcompletion.cpp
@@ -1016,7 +1016,7 @@
     }
 
     // url is malformed
-    if (!url_dir.isValid()) {
+    if (!url_dir.isValid() || url.scheme().isEmpty()) {
         return false;
     }