diff --git a/src/constrainteq.cpp b/src/constrainteq.cpp
index ef963b0..86ed312 100644
--- a/src/constrainteq.cpp
+++ b/src/constrainteq.cpp
@@ -425,8 +425,7 @@ void ConstraintBase::GenerateEquations(IdList<Equation,hEquation> *l,
 
             Expr *r = circle->CircleGetRadiusExpr();
 
-            AddEq(l,
-                ((du->Square())->Plus(dv->Square()))->Minus(r->Square()), 0);
+            AddEq(l, du->Square()->Plus(dv->Square())->Sqrt()->Minus(r), 0);
             return;
         }
 
diff --git a/test/constraint/pt_on_circle/negative_dia.slvs b/test/constraint/pt_on_circle/negative_dia.slvs
new file mode 100644
index 0000000..268c9a5
Binary files /dev/null and b/test/constraint/pt_on_circle/negative_dia.slvs differ
diff --git a/test/constraint/pt_on_circle/test.cpp b/test/constraint/pt_on_circle/test.cpp
index 5e9db1b..ce38ba5 100644
--- a/test/constraint/pt_on_circle/test.cpp
+++ b/test/constraint/pt_on_circle/test.cpp
@@ -15,3 +15,9 @@ TEST_CASE(normal_migrate_from_v22) {
     CHECK_LOAD("normal_v22.slvs");
     CHECK_SAVE("normal.slvs");
 }
+
+TEST_CASE(negative_dia) {
+    CHECK_LOAD("negative_dia.slvs");
+    Entity *e = SK.GetEntity(hRequest { 4 }.entity(0));
+    CHECK_TRUE(e->CircleGetRadiusNum() > 0);
+}