Hello, I found a type elsewhere in my code and have fixed this issue. Thanks, Matt On 27 Nov 2024, at 12:06 PM, Matthew Thomas <[email protected]> wrote: Hello, When I use MatMPIAIJSetPreallocation I get an argument out of range error, as below, [15]PETSC ERROR: Argument out of range [15]PETSC ERROR: New nonzero at (8,421) caused a malloc. Use MatSetOption(A, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE) to turn off this check However, when I run the code again with identical parameters without MatMPIAIJSetPreallocation, there is no non-zero at the location that caused the error (8, 421). Using -mat_view I can see that row 8 only contains a single value at column 8, which is expected and what I have allocated for. I have also checked that every time I call MatSetValues that this location is not being set. I am very confident my dnnz and onnz arrays have been set correctly, do you have any idea why this new non-zero is created? I am using Petsc version 3.22.1 and Slepc version 3.22.1 with fortran. This issue does not occur with small number of processors, (1-8), however, this error is consistent when I use >8 processors. Thanks, Matt