diff --git a/src/core/kprotocolmanager.cpp b/src/core/kprotocolmanager.cpp
--- a/src/core/kprotocolmanager.cpp
+++ b/src/core/kprotocolmanager.cpp
@@ -1135,6 +1135,9 @@
 
 bool KProtocolManager::isSourceProtocol(const QUrl &url)
 {
+    if (!url.isValid()) {
+        return false;
+    }
     KProtocolInfoPrivate *prot = findProtocol(url);
     if (!prot) {
         return false;
@@ -1145,6 +1148,9 @@
 
 bool KProtocolManager::supportsListing(const QUrl &url)
 {
+    if (!url.isValid()) {
+        return false;
+    }
     KProtocolInfoPrivate *prot = findProtocol(url);
     if (!prot) {
         return false;
diff --git a/src/widgets/kurlcompletion.cpp b/src/widgets/kurlcompletion.cpp
--- a/src/widgets/kurlcompletion.cpp
+++ b/src/widgets/kurlcompletion.cpp
@@ -623,6 +623,10 @@
 
     KUrlCompletionPrivate::MyURL url(text, d->cwd);
 
+    if (!url.kurl().isValid()) {
+        return QString();
+    }
+
     d->compl_text = text;
 
     // Set d->prepend to the original URL, with the filename [and ref/query] stripped.