diff --git a/include/mp/flat/nl_expr/model_api_base.h b/include/mp/flat/nl_expr/model_api_base.h
index 25ceeb104..19641dc9e 100644
--- a/include/mp/flat/nl_expr/model_api_base.h
+++ b/include/mp/flat/nl_expr/model_api_base.h
@@ -94,7 +94,7 @@ class BasicExprModelAPI
 
   /// Get linear part var \a i.
   int GetLinVar(const NLConstraint& nlc, int i) const {
-    assert(IsVarProper(i));
+    assert(IsVarProper(nlc.GetMainCon().var(i)));
     return nlc.GetMainCon().var(i);
   }