diff --git a/symforce/opt/values.cc b/symforce/opt/values.cc
index c715438f0..7538c7ec2 100644
--- a/symforce/opt/values.cc
+++ b/symforce/opt/values.cc
@@ -65,7 +65,7 @@ void Values<Scalar>::UpdateOrSet(const index_t& index, const Values<Scalar>& oth
 }
 
 template <typename Scalar>
-int32_t Values<Scalar>::NumEntries() const {
+size_t Values<Scalar>::NumEntries() const {
   return map_.size();
 }
 
diff --git a/symforce/opt/values.h b/symforce/opt/values.h
index 1dd7cff62..cc9e5f878 100644
--- a/symforce/opt/values.h
+++ b/symforce/opt/values.h
@@ -93,7 +93,7 @@ class Values {
   /**
    * Number of keys.
    */
-  int32_t NumEntries() const;
+  size_t NumEntries() const;
 
   /**
    * Has zero keys.