Addition in conditional plotting in Gnuplot forces "No usable data in this plot to auto-scale axis range"

I use a simple code to create a series of images from one file, depending on the condition. While doing so I realized that:

splot [1:150][1:150][] "dinozaur" u 5:6:(($4==0.4+0.0025*3)?$2:1/0)

does not work but:

splot [1:150][1:150][] "dinozaur" u 5:6:(($4==0.40750)?$2:1/0)

works. However, both splot [1:150][1:150][] "dinozaur" u 5:6:(($4==0.5+0.0025*3)?$2:1/0) and

splot [1:150][1:150][] "dinozaur" u 5:6:(($4==0.50750)?$2:1/0)

work. If I get an error, its the one in the title. Is there some kind of obvious mistake that I am doing ?