Addition assignment +=, also -= trigger error "attempt to subtract with underflow"?

Here's the error I'm hitting, which also shows the GLSL excerpt that triggers it:

error: proc macro panicked

   |
38 |       let gtu = glsl! {
   |  _______________^
39 | |         void main() {
40 | |             vec4 fc = vec4(1.0, 1.0, 1.0, 1.0);
41 | |             fc -= 0.1;
42 | |             gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
43 | |         }
44 | |     };
   | |_____^
   |
   = help: message: attempt to subtract with overflow

The error is the same with +=, but it is resolved if I change the shader to i.e. fc = fc - 0.1;. Are these assignment operators supported by the parser?

hadronized / glsl

Addition assignment +=, also -= trigger error "attempt to subtract with underflow"? #142