Light map Bug - Single LMs require a normal map plugged in

It seems that errors occur if no normal map texture is specified/used, even if using single lightmaps:

Shader error in 'Shader Forge/LightmappedBump Test': Shader program had errors at line 22

Shader error in 'Shader Forge/LightmappedBump Test': Program 'frag', unable to find compatible overloaded function "mul(float3x3, error)" at line 83

Keywords: DIRECTIONAL, LIGHTMAP_ON, DIRLIGHTMAP_ON, SHADOWS_OFF

Shader error in 'Shader Forge/LightmappedBump Test': Program 'frag', unable to find compatible overloaded function "saturate(error)" at line 83

Keywords: DIRECTIONAL, LIGHTMAP_ON, DIRLIGHTMAP_ON, SHADOWS_OFF

etc...