diff --git a/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in b/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
index e96ba5a1d631..8c27d2e82162 100644
--- a/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
+++ b/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
@@ -72,7 +72,8 @@ is not present on this field.
     ConstraintStrength constraintStrength( Constraint constraint ) const;
 %Docstring
 Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-is not present on this field.
+is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+for anything but ConstraintExpression when returns ConstraintStrengthHard.
 
 .. seealso:: :py:func:`constraints`
 
diff --git a/python/core/auto_generated/qgsfieldconstraints.sip.in b/python/core/auto_generated/qgsfieldconstraints.sip.in
index 51ef4a9b8ba8..f26ffa6f6e28 100644
--- a/python/core/auto_generated/qgsfieldconstraints.sip.in
+++ b/python/core/auto_generated/qgsfieldconstraints.sip.in
@@ -72,7 +72,8 @@ is not present on this field.
     ConstraintStrength constraintStrength( Constraint constraint ) const;
 %Docstring
 Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-is not present on this field.
+is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+for anything but ConstraintExpression when returns ConstraintStrengthHard.
 
 .. seealso:: :py:func:`constraints`
 
diff --git a/src/core/qgsfieldconstraints.cpp b/src/core/qgsfieldconstraints.cpp
index 371ea825b199..4954916497a3 100644
--- a/src/core/qgsfieldconstraints.cpp
+++ b/src/core/qgsfieldconstraints.cpp
@@ -29,8 +29,8 @@ QgsFieldConstraints::ConstraintStrength QgsFieldConstraints::constraintStrength(
   if ( !( mConstraints & constraint ) )
     return ConstraintStrengthNotSet;
 
-  // defaults to not set
-  return mConstraintStrengths.value( constraint, ConstraintStrengthNotSet );
+  // defaults to not set for all but expressions (which does not use strength)
+  return mConstraintStrengths.value( constraint, constraint == ConstraintExpression ? ConstraintStrengthHard : ConstraintStrengthNotSet );
 }
 
 void QgsFieldConstraints::setConstraintStrength( QgsFieldConstraints::Constraint constraint, QgsFieldConstraints::ConstraintStrength strength )
diff --git a/src/core/qgsfieldconstraints.h b/src/core/qgsfieldconstraints.h
index a5e0ee5752a8..f23b4cc5005a 100644
--- a/src/core/qgsfieldconstraints.h
+++ b/src/core/qgsfieldconstraints.h
@@ -89,7 +89,8 @@ class CORE_EXPORT QgsFieldConstraints
 
     /**
      * Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-     * is not present on this field.
+     * is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+     * for anything but ConstraintExpression when returns ConstraintStrengthHard.
      * \see constraints()
      * \see setConstraintStrength()
      */