diff --git a/gtsam_unstable/slam/PartialPriorFactor.h b/gtsam_unstable/slam/PartialPriorFactor.h
index fa06d47a3..c71ee7abd 100644
--- a/gtsam_unstable/slam/PartialPriorFactor.h
+++ b/gtsam_unstable/slam/PartialPriorFactor.h
@@ -121,7 +121,7 @@ namespace gtsam {
 
     // access
     const Vector& prior() const { return prior_; }
-    const std::vector<bool>& mask() const { return  mask_; }
+    const std::vector<size_t>& mask() const { return  mask_; }
     const Matrix& H() const { return H_; }
 
   protected: