A platform ladder apparatus having a transport position and a use position in which the apparatus is arranged to lean against a wall for performing work at an elevated level, said apparatus comprising: a ladder having a front side for climbing and a rear side facing the wall in the use position; a working platform having, seen from the front side of the ladder, a right side, a left side, a proximal side and an opposite distal side, the proximal side of the platform being pivotally connected to the ladder at a platform pivot axis located at an upper end of the ladder, the platform being arranged, in said use position of the apparatus, to be in a position, termed correct position, in relation to the wall, and to extend essentially horizontally out from the ladder with its distal side directed towards the wall, and the platform being arranged, in said transport position of the apparatus, to be pivoted about the platform pivot axis towards the ladder to a stowed position essentially parallel to the ladder; and a right and a left wall support arm for stabilizing the apparatus in the use position of the apparatus in relation to the wall, said wall support arms being connected to the platform for movement between: - a stabilizing arm position in which the wall support arms project sideways from the platform in opposite right and left directions, respectively, for stabilizing the apparatus in relation to the wall, and characterized by - a stowed arm position in which the wall support arms project out from the distal side of the platform preventing the platform from assuming said correct position in relation to the wall, thereby preventing use of the apparatus unless the support arms are brought to said stabilizing arm position for allowing the platform to assume its correct position in relation to the wall. a ladder having a front side for climbing and a rear side facing the wall in the use position; a working platform having, seen from the front side of the ladder, a right side, a left side, a proximal side and an opposite distal side, the proximal side of the platform being pivotally connected to the ladder at a platform pivot axis located at an upper end of the ladder, the platform being arranged, in said use position of the apparatus, to be in a position, termed correct position, in relation to the wall, and to extend essentially horizontally out from the ladder with its distal side directed towards the wall, and the platform being arranged, in said transport position of the apparatus, to be pivoted about the platform pivot axis towards the ladder to a stowed position essentially parallel to the ladder; and a right and a left wall support arm for stabilizing the apparatus in the use position of the apparatus in relation to the wall, said wall support arms being connected to the platform for movement between: - a stabilizing arm position in which the wall support arms project sideways from the platform in opposite right and left directions, respectively, for stabilizing the apparatus in relation to the wall, and characterized by - a stowed arm position in which the wall support arms project out from the distal side of the platform preventing the platform from assuming said correct position in relation to the wall, thereby preventing use of the apparatus unless the support arms are brought to said stabilizing arm position for allowing the platform to assume its correct position in relation to the wall. - a stabilizing arm position in which the wall support arms project sideways from the platform in opposite right and left directions, respectively, for stabilizing the apparatus in relation to the wall, and characterized by - a stowed arm position in which the wall support arms project out from the distal side of the platform preventing the platform from assuming said correct position in relation to the wall, thereby preventing use of the apparatus unless the support arms are brought to said stabilizing arm position for allowing the platform to assume its correct position in relation to the wall.