Issue when using marginsplot with splines

[Sigrid Vikjord]

Hi,

I'm plotting a survival function against BMI using cubic splines using the following code:

foreach num of numlist 3/7{
        preserve
        drop if glicopd_HUNT==.    & PartAg_gli>=40.0 & PartAg_gli!=.
        stset enddate, id(id) failure(RegisterStatus==5) origin(time PartDat_gli) scale(365.25)
        mkspline spl_bmi = BMI_gli, cubic nknots(`num') displayknots            
        stcox c.spl_bmi* i.Sex, vce(robust)
        estat ic
        levelsof BMI_gli, local(levels)
        xblc spl_bmi*, covname(BMI_gli) at(`levels') reference(25.0) eform gen(vbmi hr lb ub)
        keep if vbmi>14.9 & vbmi<40.1
        twoway rarea lb ub vbmi, lp(solid) lc(red) lw(medium) || line hr vbmi, lc(blue) lp(solid) lw(medium) ///
        legend(off) ylabel(, nogrid format(%3.2f)) yscale(log) yticks() xlabel(15(1)40, format(%3.0f)) xticks() xtitle("{bf:BMI             (kg/m{sup:2})}") ytitle("{bf:HR (95% CI) for all-cause mortality}") ///
        yline(1, lc(black) lp(dash) lw(medthin)) name(copd_death_spl_bmi_knot`num', replace) saving("M:\BMIsplines_COPD.gph",             replace)
        restore
}

I want to estimate the BMI value associated with the lowest HR. I tried using marginsplot, but got the error message spl2_bmi ambiguous abbreviation
r(111); using the code below. What do I do wrong? And is there a way to estimate a 95% CI around the BMI associated with lowest mortality?

foreach num of numlist 3/7 {
    preserve
    drop if glicopd_HUNT == . & PartAg_gli >= 40.0 & PartAg_gli != .
    stset enddate, id(id) failure(RegisterStatus == 5) origin(time PartDat_gli) scale(365.25)
    mkspline spl_bmi = BMI_gli, cubic nknots(`num') displayknots
    stcox c.spl_bmi* i.Sex, vce(robust)
    margins, at("spl_bmi") post
    qui marginsplot
    // Extract the lowest point
    qui summarize spl_bmi if _bs_1 == min(_bs_1) & e(sample)
    di "Lowest BMI value: " r(mean)
    restore
}

* Example generated by -dataex-. For more info, type help dataex
clear
input float(glicopd_HUNT PartAg_gli enddate_bmi) double(RegisterStatus Sex) float(BMI_gli id)
3 24.2 23481 1 1   30   1
3 29.2 23481 1 0 32.3   2
3 43.8 23481 1 0 32.3   3
3 73.7 23481 1 0 24.4   4
1 73.6 23481 5 0 23.1   5
1 52.4 23481 1 1 27.3   6
3 52.3 23481 1 1 21.8   7
3 76.2 23481 1 0 24.1   8
3 79.9 23481 1 0 24.3   9
2 37.5 23481 1 0 22.6  10
3 60.6 23481 1 0 23.4  11
1 34.3 23481 1 1   28  12
3 74.6 23481 1 1 26.1  13
3 73.9 23481 1 0 34.4  14
3 63.4 23481 1 0 30.1  15
1 42.6 23481 1 1 27.2  16
2 52.8 23481 1 1 30.2  17
1 24.3 23481 1 1   25  18
3 73.5 23481 1 0 25.1  19
3 64.9 23481 1 0 34.9  20
3 45.6 23481 1 0 22.3  21
3 32.8 23481 1 0 22.4  22
1 40.6 23481 1 0 24.8  23
3 42.2 23481 1 0 21.2  24
3 27.8 23481 1 0 26.3  25
2 40.8 23481 1 1   26  26
3 41.7 23481 1 1 25.8  27
1 25.1 23481 1 1   24  28
3 66.8 23481 1 0 28.7  29
3 30.6 23481 1 0 24.1  30
3 55.3 23481 1 0 24.6  31
3 47.3 23481 1 1 26.2  32
1 58.6 14440 5 1 23.6  33
2 19.5 23481 1 0 27.7  34
3 33.4 23481 1 1 23.4  35
1 62.1 23481 5 0 29.5  36
3 64.4 23481 1 0 24.2  37
3 52.6 23481 1 1 34.9  38
3 72.7 23481 1 0 20.5  39
1 59.5 21015 5 1 31.4  40
1 31.7 23481 1 1 22.6  41
3   49 23481 1 1 23.3  42
2 35.3 23481 1 0 28.7  43
1   77 14898 5 0 26.1  44
2 40.7 23481 1 0 21.9  45
3 32.2 23481 1 1   28  46
3 26.1 23481 1 0 33.6  47
1 40.2 23481 1 1 22.3  48
3 57.2 23481 1 0 33.9  49
3 60.7 23481 1 0   29  50
3   65 23481 1 0 28.6  51
1 49.8 23481 5 1 27.4  52
2 72.8 23481 5 0   25  53
1 67.7 18489 5 1   27  54
3 25.7 23481 1 0 24.2  55
2   20 23481 1 0 20.7  56
3 46.6 23481 1 1 25.3  57
1   42 23481 1 1 23.7  58
3 43.3 23481 1 1   28  59
3 45.6 23481 1 0 22.3  60
3 67.8 23481 1 1 26.3  61
3 39.1 23481 1 1 24.9  62
1 26.7 23481 1 1 23.9  63
1 63.9 22415 5 0 23.5  64
3 46.2 23481 1 0 22.5  65
3 26.7 23481 1 0 24.1  66
3 62.5 23481 1 1 21.9  67
3 32.7 23481 1 0 23.3  68
1 49.4 23481 1 1 26.6  69
1 42.4 23481 1 1 29.1  70
3 78.5 23481 1 1 26.6  71
3 69.2 23481 1 1 32.1  72
1 74.3 17454 5 1 27.6  73
3 44.2 23481 1 0 27.7  74
1 47.7 23481 1 0 24.2  75
3 40.8 23481 1 0 30.2  76
3 66.9 23481 1 0 23.8  77
3 73.5 23481 1 1 31.4  78
1 54.2 18611 5 0 25.6  79
2 55.9 23481 1 1 21.9  80
1 42.6 23481 1 0 24.3  81
3 34.7 23481 1 0   32  82
3 32.7 23481 1 0 29.3  83
3 43.2 23481 1 1 25.5  84
1   32 23481 1 0 19.9  85
1 76.8 14075 5 0   24  86
1 59.9 19342 5 1 26.6  87
2 29.3 23481 1 1 20.5  88
1 47.7 23481 1 1 24.2  89
3 33.2 23481 1 0   30  90
3 32.2 23481 1 1 25.5  91
3 32.9 23481 1 0 27.5  92
1 57.7 18732 5 1 32.1  93
3 50.1 23481 1 0 27.3  94
3 74.1 23481 1 0 20.4  95
1 36.3 23481 1 1 24.7  96
2 27.6 23481 1 0 21.3  97
3 22.7 23481 1 0 19.7  98
3 45.7 23481 1 1 28.5  99
3 44.9 23481 1 1 28.5 100
end
format %td enddate_bmi
label values RegisterStatus RegisterStatus
label def RegisterStatus 1 "Bosatt", modify
label def RegisterStatus 5 "Død", modify
label values Sex Sex
label def Sex 0 "Kvinne", modify
label def Sex 1 "Mann", modify