Distribution of Macroscopic Electric Charge within Insulator

Theorem
Let $B$ be a body made out of an electrically insulating substance.

Then it is possible for different volume elements of $B$ to have different (macroscopic) electric fields caused by intrinsic imbalance of the electric charges distributed throughout $B$.

Proof
In an electrical insulator, electric charges cannot flow, because of the nature of the substance.

Hence an electric charge in one point within $B$ cannot flow to another point within $B$ under a potential difference caused by the presence of those (macroscopic) electric fields.