fontTools.varLib.errors.VarLibValidationError: Missing glyphs are referenced in conditional substitution rules: Bold, static/ff/font3.ttf

[zhenzi0322]

my1.designspace：

<designspace>
    <axes>
        <axis tag="wght" name="Weight" minimum="100" maximum="900" default="400"/>
        <axis tag="wdth" name="Width" minimum="0" maximum="1000" default="500"/>
    </axes>
    <sources>
        <source familyname="SourceFont" filename="static/ff/font1.ttf" stylename="Regular">
            <location>
                <dimension name="Weight" xvalue="400"/>
                <dimension name="Width" xvalue="500"/>
            </location>
        </source>
        <source familyname="SourceFont1" filename="static/ff/font3.ttf" stylename="Bold">
            <location>
                <dimension name="Weight" xvalue="700"/>
                <dimension name="Width" xvalue="800"/>
            </location>
        </source>
    </sources>
    <instances>
        <instance familyname="SourceFont" stylename="Regular" filename="static/ff/font1.ttf">
            <location>
                <dimension name="Weight" xvalue="400"/>
                <dimension name="Width" xvalue="500"/>
            </location>
            <stylename>Regular</stylename>
        </instance>
        <instance familyname="SourceFont1" stylename="Bold" filename="static/ff/font3.ttf">
            <location>
                <dimension name="Weight" xvalue="700"/>
                <dimension name="Width" xvalue="800"/>
            </location>
            <stylename>Bold</stylename>
        </instance>
    </instances>
    <rules>
        <rule name="Switch to Bold if Weight is 700 or greater">
            <conditionset>
                <condition minimum="700" name="Weight"/>
            </conditionset>
            <sub name="Bold" with="static/ff/font3.ttf"/>
        </rule>
    </rules>
</designspace>

test.py：

from fontTools.varLib import build
from fontTools.varLib.instancer import instantiateVariableFont
design_space_path = "my1.designspace"

font = build(design_space_path)
instanceFont = instantiateVariableFont(font[0], axisLimits={"wght": (400, 700), "wdth": (500, 800)}, inplace=True)
instanceFont.save("my_variable_font.ttf")

errror:

ile "E:\environment\python\Envs\demo\lib\site-packages\fontTools\varLib\featureVars.py", line 89, in _checkSubstitutionGlyphsExist
    raise VarLibValidationError(
fontTools.varLib.errors.VarLibValidationError: Missing glyphs are referenced in conditional substitution rules: Bold, static/ff/font3.ttf

May I ask why this error is reported? Shouldn't I switch fonts?

[anthrotype]

DesignSpace rules switch glyphs, not fonts.

<sub name="Bold" with="static/ff/font3.ttf"/>

"static/ff/font3.ttf" is not a glyph name, and neither "Bold" looks like one

[zhenzi0322]

How to switch fonts

[justvanrossum]

That is not a concept that exists in .designspace / OpenType.

[zhenzi0322]

How to generate variable fonts? The variable font I tried to generate has a consistent font style.