diff --git a/include/constants.h b/include/constants.h
index 621c955d..c3a26723 100644
--- a/include/constants.h
+++ b/include/constants.h
@@ -8,11 +8,13 @@
 #include <petsc.h>
 #define freq 60.0                   /**< System frequency */
 #define w_s (2.0 * PETSC_PI * freq) /**< Angular speed */
-#define MAXLINE 10000               /**< Max. number of characters in a line */
-#define ISOLATED_BUS 4              /**< Isolated bus */
-#define REF_BUS 3                   /**< Reference bus (swing bus) */
-#define PV_BUS 2                    /**< PV (voltage-controlled) bus */
-#define PQ_BUS 1                    /**< PQ bus */
+// FIXME: either validate that a line fits within this buffer or replace the use
+//        of a fixed-size buffer
+#define MAXLINE 1000000 /**< Max. number of characters in a line */
+#define ISOLATED_BUS 4  /**< Isolated bus */
+#define REF_BUS 3       /**< Reference bus (swing bus) */
+#define PV_BUS 2        /**< PV (voltage-controlled) bus */
+#define PQ_BUS 1        /**< PQ bus */
 #define NGEN_AT_BUS_MAX                                                        \
   32                        /**< Maximum number of generators allowed at a bus \
                              */
diff --git a/src/tcopflow/interface/tcopflow.cpp b/src/tcopflow/interface/tcopflow.cpp
index 37d18449..bc0413fb 100644
--- a/src/tcopflow/interface/tcopflow.cpp
+++ b/src/tcopflow/interface/tcopflow.cpp
@@ -527,9 +527,9 @@ PetscErrorCode TCOPFLOWSetUp(TCOPFLOW tcopflow) {
   CHKERRQ(ierr);
   ierr = MatSetFromOptions(tcopflow->Jac);
   CHKERRQ(ierr);
-  /* Assume 10% sparsity */
-  ierr = MatSeqAIJSetPreallocation(tcopflow->Jac,
-                                   (PetscInt)(0.1 * tcopflow->Nx), NULL);
+  /* Assume 10% sparsity with a minimum of 50 nz per row */
+  PetscInt jac_nz_per_row = PetscMin((PetscInt)(0.1 * tcopflow->Nx), 50);
+  ierr = MatSeqAIJSetPreallocation(tcopflow->Jac, jac_nz_per_row, NULL);
   CHKERRQ(ierr);
   ierr =
       MatSetOption(tcopflow->Jac, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE);
@@ -543,9 +543,9 @@ PetscErrorCode TCOPFLOWSetUp(TCOPFLOW tcopflow) {
   CHKERRQ(ierr);
   ierr = MatSetFromOptions(tcopflow->Hes);
   CHKERRQ(ierr);
-  /* Assume 10% sparsity */
-  ierr = MatSeqAIJSetPreallocation(tcopflow->Hes,
-                                   (PetscInt)(0.1 * tcopflow->Nx), NULL);
+  /* Assume 10% sparsity with a minimum of 50 nz per row */
+  PetscInt hes_nz_per_row = PetscMin((PetscInt)(0.1 * tcopflow->Nx), 50);
+  ierr = MatSeqAIJSetPreallocation(tcopflow->Hes, hes_nz_per_row, NULL);
   CHKERRQ(ierr);
   ierr =
       MatSetOption(tcopflow->Hes, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE);