diff --git a/src/engine/engine_util_spatial.h b/src/engine/engine_util_spatial.h
index c90cb27f..e704facb 100644
--- a/src/engine/engine_util_spatial.h
+++ b/src/engine/engine_util_spatial.h
@@ -102,7 +102,7 @@ void mju_inertCom(mjtNum res[10], const mjtNum inert[3], const mjtNum mat[9],
 void mju_dofCom(mjtNum res[6], const mjtNum axis[3], const mjtNum offset[3]);
 
 // multiply 6D vector (rotation, translation) by 6D inertia matrix
-MJAPI void mju_mulInertVec(mjtNum res[6], const mjtNum inert[10], const mjtNum vec[6]);
+MJAPI void mju_mulInertVec(mjtNum* res, const mjtNum inert[10], const mjtNum vec[6]);
 
 // multiply dof matrix by vector
 void mju_mulDofVec(mjtNum* res, const mjtNum* mat, const mjtNum* vec, int n);