'str' object has no attribute 'append'

I am trying to compile a variable font from DesignSpace + UFO files, and getting the following output.

(env) E:\76\googlefonts\KumbhSans\1.0.0.3\sources>fontmake -o variable -m KumbhSans_Var_35.designspace --verbose DEBUG --validate-ufo

DEBUG:fontmake.font_project:ufoLib UFO validation is enabled

INFO:fontmake.font_project:Building variable font variable_ttf\KumbhSans_Var_35-VF.ttf

INFO:ufo2ft:Pre-processing glyphs

INFO:cu2qu.ufo:New spline lengths: 1: 242, 2: 1036, 3: 725, 4: 167, 5: 1

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-Black

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-BlackDisplay

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-Display

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-Regular

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-Thin

INFO:ufo2ft.filters.base:Running DecomposeComponentsFilter on KumbhSans-ThinDisplay

INFO:ufo2ft:Building OpenType tables for KumbhSans-Black

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building OpenType tables for KumbhSans-BlackDisplay

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building OpenType tables for KumbhSans-Display

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building OpenType tables for KumbhSans-Regular

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building OpenType tables for KumbhSans-Thin

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building OpenType tables for KumbhSans-ThinDisplay

DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No dist-enabled scripts defined in languagesystem statements; dist feature will not be generated

DEBUG:ufo2ft.featureWriters.gdefFeatureWriter.GdefFeatureWriter:No features to be generated; skipped

INFO:ufo2ft:Building variable TTF font

INFO:fontTools.varLib:Axes:

[{'default': 400.0,

  'hidden': False,

  'labelNames': {'en': 'Weight'},

  'map': [(100.0, 100.0), (400.0, 400.0), (900.0, 900.0)],

  'maximum': 900.0,

  'minimum': 100.0,

  'name': 'weight',

  'tag': 'wght'},

 {'default': 8.0,

  'hidden': False,

  'labelNames': {'en': 'Optical Size'},

  'map': [],

  'maximum': 72.0,

  'minimum': 8.0,

  'name': 'optical size',

  'tag': 'opsz'}]

INFO:fontTools.varLib:Internal master locations:

[{'optical size': 8.0, 'weight': 900.0},

 {'optical size': 72.0, 'weight': 900.0},

 {'optical size': 72.0, 'weight': 400.0},

 {'optical size': 8.0, 'weight': 400.0},

 {'optical size': 8.0, 'weight': 100.0},

 {'optical size': 72.0, 'weight': 100.0}]

INFO:fontTools.varLib:Internal axis supports:

{'optical size': [8.0, 8.0, 72.0], 'weight': [100.0, 400.0, 900.0]}

INFO:fontTools.varLib:Normalized master locations:

[{'optical size': 0.0, 'weight': 1.0},

 {'optical size': 1.0, 'weight': 1.0},

 {'optical size': 1.0, 'weight': 0.0},

 {'optical size': 0.0, 'weight': 0.0},

 {'optical size': 0.0, 'weight': -1.0},

 {'optical size': 1.0, 'weight': -1.0}]

INFO:fontTools.varLib:Index of base master: 3

INFO:fontTools.varLib:Building variable font

INFO:fontTools.varLib:Loading master fonts

INFO:fontTools.varLib:Generating fvar

INFO:fontTools.varLib:Generating avar

INFO:fontTools.varLib:No need for avar

INFO:fontTools.varLib:Building variations tables

INFO:fontTools.varLib:Generating MVAR

INFO:fontTools.varLib:  hdsc: OS/2.sTypoDescender       [-520, -520, -533, -533, -540, -540]

INFO:fontTools.varLib:  hcld: OS/2.usWinDescent [918, 918, 918, 918, 918, 938]

INFO:fontTools.varLib:Generating HVAR

INFO:fontTools.varLib:Merging OpenType Layout tables

ERROR:root:In 'KumbhSans_Var_35.designspace': Generating fonts from Designspace failed: 'str' object has no attribute 'append'

Traceback (most recent call last):

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 93, in mergeObjects

    mergerFunc(self, value, values)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 159, in merge

    raise ShouldBeConstant(self, expected=allValues[0], got=lst, stack="."+k)

fontTools.varLib.errors.ShouldBeConstant: 


During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "e:\76\googlefonts\env\lib\site-packages\fontmake\font_project.py", line 960, in run_from_designspace

    self._run_from_designspace_interpolatable(

  File "e:\76\googlefonts\env\lib\site-packages\fontmake\font_project.py", line 1040, in _run_from_designspace_interpolatable

    self.build_variable_font(

  File "e:\76\googlefonts\env\lib\site-packages\fontmake\font_project.py", line 331, in build_variable_font

    font = ufo2ft.compileVariableTTF(

  File "e:\76\googlefonts\env\lib\site-packages\ufo2ft\__init__.py", line 648, in compileVariableTTF

    varfont = varLib.build(

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\__init__.py", line 926, in build

    _merge_OTL(vf, model, master_fonts, axisTags)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\__init__.py", line 619, in _merge_OTL

    merger.mergeTables(font, master_fonts, ['GSUB', 'GDEF', 'GPOS'])

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 130, in mergeTables

    self.mergeThings(font[tag], [m[tag] if tag in m else None

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 1023, in mergeThings

    super(VariationMerger, self).mergeThings(out, lst)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 118, in mergeThings

    self.mergeObjects(out, lst)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 93, in mergeObjects

    mergerFunc(self, value, values)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 1023, in mergeThings

    super(VariationMerger, self).mergeThings(out, lst)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 118, in mergeThings

    self.mergeObjects(out, lst)

  File "e:\76\googlefonts\env\lib\site-packages\fontTools\varLib\merger.py", line 95, in mergeObjects

    e.stack.append('.'+key)

AttributeError: 'str' object has no attribute 'append'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):

  File "e:\76\googlefonts\env\lib\site-packages\fontmake\__main__.py", line 501, in main

    project.run_from_designspace(designspace_path, **args)

  File "e:\76\googlefonts\env\lib\site-packages\fontmake\font_project.py", line 974, in run_from_designspace

    raise FontmakeError(

fontmake.errors.FontmakeError: In 'KumbhSans_Var_35.designspace': Generating fonts from Designspace failed: 'str' object has no attribute 'append'

All glyphs are checked for equal number of nodes and compatibility.
Each glyph works fine in variations when checked in Fontlab.

While exporting the DesignSpace + UFO from Fontlab, I have tried with different settings like enabling/disabling mark/mkmk code. Toggling "remove overlaps, nodes at extreme, countour direction, cleanup, etc.".

Thanks in advance for any help or suggestion.