Cannot get rid of nuget warning NU5128 about missing dependency group

[Env: nuget.exe 5.9.1.11, VS Pro 2019 16.9.6, W10 x64 1909]

I am having trouble configuring the dependencies section of my .nuspec file so nuget generates a package which correctly contains the Dependency for .NET Framework 4.7.2. I consistently get the warning

WARNING: NU5128: Some target frameworks declared in the dependencies group of the nuspec and thelib/ref folder do not have exact matches in the other location. Consult the list of actions below:- Add a dependency group for .NETFramework4.7.2 to the nuspec

when I run the nuget pack command. In the .nupkg archive, there are lib\net472\MyLib.dll and lib\net472\MyLib.xml files generated by nuget. (I am running nuget in the VS project directory which contains a .csproj file and my .nuspec file.) Here are the different configurations of the dependencies section that I have tried (my library does have a dependency on the MathNet.Numerics NuGet package):

<dependencies><group targetFramework=".NETFramework4.7.2" /></dependencies>

&

<dependencies><group targetFramework=".NETFramework4.7.2" /><group><dependency id="MathNet.Numerics" version="4.15.0" /></group></dependencies>

&

<dependencies><group><dependency id="MathNet.Numerics" version="4.15.0" /></group><group targetFramework=".NETFramework4.7.2" /></dependencies>

&

<dependencies><group><dependency id="MathNet.Numerics" version="4.15.0" /></group><group targetFramework="net472" /></dependencies>

&

<dependencies><group targetFramework="net472" /><group><dependency id="MathNet.Numerics" version="4.15.0" /></group></dependencies>

&

<dependencies><group targetFramework="net472" /></dependencies>

&

<dependencies><group targetFramework="net472"><dependency id="MathNet.Numerics" version="4.15.0" /></group></dependencies>

&

<dependencies><group targetFramework=".NETFramework4.7.2"><dependency id="MathNet.Numerics" version="4.15.0" /></group></dependencies>

& no <dependencies></dependencies> section as well.