diff --git a/inference/dimmwitted/src/factor_graph.cc b/inference/dimmwitted/src/factor_graph.cc
index 2b713d1da..f036adc74 100644
--- a/inference/dimmwitted/src/factor_graph.cc
+++ b/inference/dimmwitted/src/factor_graph.cc
@@ -158,6 +158,8 @@ void FactorGraph::construct_index_part(size_t v_start, size_t v_end,
           v.total_truthiness += item.second.truthiness;
         }
         for (size_t j = 0; j < v.cardinality; ++j) {
+          // catch spurious values (e.g., due to duplicate tuples from DD)
+          assert(value_list[j] != Variable::INVALID_VALUE);
           values[value_index_base] =
               VariableToFactor(value_list[j], truthiness_list[j], 0, 0);
           ++value_index_base;
diff --git a/inference/dimmwitted/src/inference_result.h b/inference/dimmwitted/src/inference_result.h
index afe47c40e..e20ecec06 100644
--- a/inference/dimmwitted/src/inference_result.h
+++ b/inference/dimmwitted/src/inference_result.h
@@ -64,8 +64,7 @@ class InferenceResult {
   void dump_marginals_in_text(std::ostream &text_output) const;
 
   inline void update_weight(size_t wid, double stepsize, double gradient) {
-    double diff = stepsize * gradient;
-    weight_grads[wid] += diff;
+    weight_grads[wid] += gradient;
     double weight = weight_values[wid];
     switch (opts.regularization) {
       case REG_L2: {
@@ -80,7 +79,7 @@ class InferenceResult {
       default:
         std::abort();
     }
-    weight -= diff;
+    weight -= stepsize * gradient;
     weight_values[wid] = weight;
   }