diff --git a/ext/NLoptMathOptInterfaceExt.jl b/ext/NLoptMathOptInterfaceExt.jl
index 1db0261..180d24f 100644
--- a/ext/NLoptMathOptInterfaceExt.jl
+++ b/ext/NLoptMathOptInterfaceExt.jl
@@ -587,7 +587,7 @@ function _fill_gradient(grad, x, f::MOI.ScalarQuadraticFunction{Float64})
     end
     for term in f.quadratic_terms
         i, j = term.variable_1.value, term.variable_2.value
-        grad[i] += term.coefficient * x[i]
+        grad[i] += term.coefficient * x[j]
         if i != j
             grad[j] += term.coefficient * x[i]
         end