diff --git a/src/ksanewidget.h b/src/ksanewidget.h
--- a/src/ksanewidget.h
+++ b/src/ksanewidget.h
@@ -211,7 +211,8 @@
 
     /** This method can be used to write many parameter values at once.
      * @param opts is a QMap with the parameter names and values.
-     * @return This function returns the number of successful writes. */
+     * @return This function returns the number of successful writes
+     * or -1 if scanning is in progress. */
     int setOptVals(const QMap <QString, QString> &opts);
 
     /** This function reads one parameter value into a string.
@@ -223,7 +224,8 @@
     /** This function writes one parameter value into a string.
      * @param optname is the name of the parameter to write.
      * @param value is the string representation of the value.
-     * @return this function returns true if the write was successful. */
+     * @return this function returns true if the write was successful and
+     * false if it was unsuccessful or scanning is in progress. */
     bool setOptVal(const QString &optname, const QString &value);
 
     /** This function sets the label on the final scan button
diff --git a/src/ksanewidget.cpp b/src/ksanewidget.cpp
--- a/src/ksanewidget.cpp
+++ b/src/ksanewidget.cpp
@@ -718,6 +718,11 @@
 
 int KSaneWidget::setOptVals(const QMap <QString, QString> &opts)
 {
+    if (d->m_scanThread->isRunning() ||
+            d->m_previewThread->isRunning()) {
+        return -1;
+    }
+
     QString tmp;
     int i;
     int ret = 0;
@@ -766,6 +771,11 @@
 
 bool KSaneWidget::setOptVal(const QString &option, const QString &value)
 {
+    if (d->m_scanThread->isRunning() ||
+            d->m_previewThread->isRunning()) {
+        return false;
+    }
+
     KSaneOption *opt;
 
     if ((opt = d->getOption(option)) != nullptr) {