diff --git a/doc/includes/references.h b/doc/includes/references.h
index 6aaec67d..0887af0b 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -97,7 +97,7 @@ typedef struct mjData_ {
   // solver statistics
   mjSolverStat  solver[mjNISLAND*mjNSOLVER];  // solver statistics per island, per iteration
   int           solver_niter[mjNISLAND];      // number of solver iterations, per island
-  int           solver_nnz[mjNISLAND];        // number of nonzeros in Hessian or efc_AR, per island
+  int           solver_nnz[mjNISLAND];        // number of nonzeros in solver matrix, per island
   mjtNum        solver_fwdinv[2];             // forward-inverse comparison: qfrc, efc
 
   // diagnostics
diff --git a/include/mujoco/mjdata.h b/include/mujoco/mjdata.h
index f31dc5b6..d95d4e5a 100644
--- a/include/mujoco/mjdata.h
+++ b/include/mujoco/mjdata.h
@@ -121,7 +121,7 @@ typedef struct mjData_ {
   // solver statistics
   mjSolverStat  solver[mjNISLAND*mjNSOLVER];  // solver statistics per island, per iteration
   int           solver_niter[mjNISLAND];      // number of solver iterations, per island
-  int           solver_nnz[mjNISLAND];        // number of nonzeros in Hessian or efc_AR, per island
+  int           solver_nnz[mjNISLAND];        // number of nonzeros in solver matrix, per island
   mjtNum        solver_fwdinv[2];             // forward-inverse comparison: qfrc, efc
 
   // diagnostics
diff --git a/python/mujoco/introspect/structs.py b/python/mujoco/introspect/structs.py
index 1fe6ce1f..7e281505 100644
--- a/python/mujoco/introspect/structs.py
+++ b/python/mujoco/introspect/structs.py
@@ -5524,7 +5524,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                      inner_type=ValueType(name='int'),
                      extents=(20,),
                  ),
-                 doc='number of nonzeros in Hessian or efc_AR, per island',
+                 doc='number of nonzeros in solver matrix, per island',
              ),
              StructFieldDecl(
                  name='solver_fwdinv',
diff --git a/src/engine/engine_solver.c b/src/engine/engine_solver.c
index e3f9f174..cf81f039 100644
--- a/src/engine/engine_solver.c
+++ b/src/engine/engine_solver.c
@@ -2474,7 +2474,7 @@ static void mj_solPrimal(const mjModel* m, mjData* d, int island, int maxiter, i
         d->solver_nnz[island_stat] = nv*nv;
       }
     } else {
-      d->solver_nnz[island_stat] = 0;
+      d->solver_nnz[island_stat] = ctx.nJ;
     }
   }