"FOR without a matching NEXT" even though there is a next for it.

[selbstdual]

Dear readers,
I wrote the following

    for counter_outside = 1 to ((Y_top - Y_below)/steps)
        for counter = 1 to ((X_right - X_left))
            if right_left = 1 then
                GOSUB Turn_left
                right_left = 0
            else
                GOSUB Turn_right
                right_left = 1
            endIF
            Pause 20
        NEXT counter
        
        for counter = 1 to steps
            if top_below = 0 then
                GOSUB Turn_below
            else
                GOSUB Turn_top
            end
            Pause 20
        NEXT counter
        
    NEXT counter_outside

and microcode studio says "FOR without a matching NEXT." and marks counter_outside. What is wrong ?