diff --git a/src/libcalamares/Settings.cpp b/src/libcalamares/Settings.cpp
index 0a475be00..e5bc71c97 100644
--- a/src/libcalamares/Settings.cpp
+++ b/src/libcalamares/Settings.cpp
@@ -73,6 +73,16 @@ requireBool( const YAML::Node& config, const char* key, bool d )
 namespace Calamares
 {
 
+InstanceDescription::InstanceDescription( const Calamares::ModuleSystem::InstanceKey& key )
+    : m_instanceKey( key )
+    , m_weight( 1 )
+{
+    if ( !isValid() )
+    {
+        m_weight = 0;
+    }
+}
+
 InstanceDescription::InstanceDescription( Calamares::ModuleSystem::InstanceKey&& key, int weight )
     : m_instanceKey( key )
     , m_weight( qBound( 1, weight, 100 ) )
@@ -245,6 +255,50 @@ Settings::Settings( const QString& settingsFilePath, bool debugMode )
     s_instance = this;
 }
 
+void
+Settings::validateSequence()
+{
+    // Since moduleFinder captures targetKey by reference, we can
+    //   update targetKey to change what the finder lambda looks for.
+    Calamares::ModuleSystem::InstanceKey targetKey;
+    auto moduleFinder = [&targetKey]( const InstanceDescription& d ) { return d.isValid() && d.key() == targetKey; };
+
+    // Check the sequence against the existing instances (which so far are only custom)
+    for ( const auto& step : m_modulesSequence )
+    {
+        for ( const auto& instance : step.second )
+        {
+            Calamares::ModuleSystem::InstanceKey k = Calamares::ModuleSystem::InstanceKey::fromString( instance );
+            if ( !k.isValid() )
+            {
+                cWarning() << "Invalid instance key in *sequence*," << instance;
+            }
+            if ( k.isValid() && k.isCustom() )
+            {
+                targetKey = k;
+                const auto it = std::find_if(
+                    m_customModuleInstances.constBegin(), m_customModuleInstances.constEnd(), moduleFinder );
+                if ( it == m_customModuleInstances.constEnd() )
+                {
+                    cWarning() << "Custom instance key" << instance << "is not listed in the *instances*";
+                    // don't add it, let this fail later.
+                }
+            }
+            if ( k.isValid() && !k.isCustom() )
+            {
+                targetKey = k;
+                const auto it = std::find_if(
+                    m_customModuleInstances.constBegin(), m_customModuleInstances.constEnd(), moduleFinder );
+                if ( it == m_customModuleInstances.constEnd() )
+                {
+                    // Non-custom instance, just mentioned in *sequence*
+                    m_customModuleInstances.append( InstanceDescription( k ) );
+                }
+            }
+        }
+    }
+}
+
 void
 Settings::setConfiguration( const QByteArray& ba, const QString& explainName )
 {
@@ -265,6 +319,8 @@ Settings::setConfiguration( const QByteArray& ba, const QString& explainName )
         m_disableCancel = requireBool( config, "disable-cancel", false );
         m_disableCancelDuringExec = requireBool( config, "disable-cancel-during-exec", false );
         m_quitAtEnd = requireBool( config, "quit-at-end", false );
+
+        validateSequence();
     }
     catch ( YAML::Exception& e )
     {
diff --git a/src/libcalamares/Settings.h b/src/libcalamares/Settings.h
index de12ab57a..97ee77bc4 100644
--- a/src/libcalamares/Settings.h
+++ b/src/libcalamares/Settings.h
@@ -59,6 +59,16 @@ public:
      */
     InstanceDescription() = default;
 
+    /** @brief An InstanceDescription with no special settings.
+     *
+     * Regardless of @p key being custom, sets weight to 1 and
+     * the configuration file to @c key.module() (plus the ".conf"
+     * extension).
+     *
+     * To InstanceDescription is custom if the key is.
+     */
+    InstanceDescription( const InstanceKey& key );
+
     static InstanceDescription fromSettings( const QVariantMap& );
 
     bool isValid() const { return m_instanceKey.isValid(); }
@@ -84,6 +94,7 @@ public:
     explicit Settings( const QString& settingsFilePath, bool debugMode );
 
     void setConfiguration( const QByteArray& configData, const QString& explainName );
+    void validateSequence();
 
 public:
     static Settings* instance();