diff --git a/src/frontends/qt/Validator.cpp b/src/frontends/qt/Validator.cpp
index 0d3b672f12..1ef90ac2c8 100644
--- a/src/frontends/qt/Validator.cpp
+++ b/src/frontends/qt/Validator.cpp
@@ -56,6 +56,10 @@ QValidator::State LengthValidator::validate(QString & qtext, int &) const
 	if (ok && unsigned_ && d < 0)
 		return QValidator::Invalid;
 
+	if (ok && positive_ && d == 0)
+		// A plausible intermediate value, see #12508
+		return QValidator::Intermediate;
+
 	if (ok && positive_ && d <=0)
 		return QValidator::Invalid;