imageStore wants ivec4 instead of an int

I have the following fragment shader code:

in vec4 gl_FragCoord;

out vec4 out_col;

uniform layout(binding = 0, offset = 0) atomic_uint ac;
uniform layout(r32i) iimageBuffer head_buffer;
uniform layout(rgba32f) imageBuffer color_buffer;
uniform layout(r32i) iimageBuffer next_buffer;

void main()
{
    int id = int(gl_FragCoord.x + gl_FragCoord.y * 1024);
    uint index = atomicCounterIncrement(ac);
    int oldHeader = imageAtomicExchange(head_buffer, id, int(index));
    imageStore(next_buffer, int(index), ivec4(oldHeader, 0, 0, 0)); // <----- !!! !!!
    imageStore(color_buffer, int(index), vec4(diffuse().rgb, gl_FragCoord.z));

    out_col = vec4(1.0, 1.0, 1.0, 1.0);
}

When passing the value to be set to imageStore() as ivec4

imageStore(next_buffer, int(index), ivec4(oldHeader, 0, 0, 0));

there is no error message.

When passing the value to be set to imageStore() as int

imageStore(next_buffer, int(index), oldHeader);

there is the following OpenGL error message at runtime:

0(89) : error C7011: implicit cast from “int” to “ivec4”

BUT WHY - I thought “next_buffer” would be an “r32i” (=integer) buffer?
What am I doing wrong?