Try changing uvec4 to vec4. The documentation for mix is here:
According to the OpenGL documentation, genType can be float vec2 vec3 or vec4. There are no prototypes in mix() for genUType, which uvec4 is part of.
Hope this helps,
Ok, missed that. It is also in the QuickRef Card. Tu is for unsigned types.
I was a bit tunnel visioned, because for relational functions, T also means Tu...