diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e2220d..63ad6b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,8 @@ New sketch features:
     drag the point from the source sketch.
   * When dragging an arc or rectangle point, it will be automatically
     constrained to other points with a click.
+  * When adding a constraint which has a label and is redundant with another
+    constraint, the constraint is added as a reference, avoiding an error.
 
 New export/import features:
   * Three.js: allow configuring projection for exported model, and initially
diff --git a/src/constraint.cpp b/src/constraint.cpp
index 14e3271..b1f5d1f 100644
--- a/src/constraint.cpp
+++ b/src/constraint.cpp
@@ -733,6 +733,14 @@ void Constraint::MenuConstrain(Command id) {
         default: ssassert(false, "Unexpected menu ID");
     }
 
+    if(SK.constraint.FindByIdNoOops(c.h)) {
+        Constraint *constraint = SK.GetConstraint(c.h);
+        if(SS.TestRankForGroup(c.group) == SolveResult::REDUNDANT_OKAY &&
+           constraint->HasLabel()) {
+            constraint->reference = true;
+        }
+    }
+
     SS.GW.ClearSelection();
     InvalidateGraphics();
 }
diff --git a/src/generate.cpp b/src/generate.cpp
index a89f59b..fc5769b 100644
--- a/src/generate.cpp
+++ b/src/generate.cpp
@@ -498,7 +498,7 @@ void SolveSpaceUI::SolveGroupAndReport(hGroup hg, bool andFindFree) {
     }
 }
 
-void SolveSpaceUI::SolveGroup(hGroup hg, bool andFindFree) {
+void SolveSpaceUI::WriteEqSystemForGroup(hGroup hg) {
     int i;
     // Clear out the system to be solved.
     sys.entity.Clear();
@@ -528,6 +528,11 @@ void SolveSpaceUI::SolveGroup(hGroup hg, bool andFindFree) {
     }
 
     MarkDraggedParams();
+}
+
+void SolveSpaceUI::SolveGroup(hGroup hg, bool andFindFree) {
+    WriteEqSystemForGroup(hg);
+    Group *g = SK.GetGroup(hg);
     g->solved.remove.Clear();
     SolveResult how = sys.Solve(g, &(g->solved.dof),
                                    &(g->solved.remove),
@@ -541,6 +546,15 @@ void SolveSpaceUI::SolveGroup(hGroup hg, bool andFindFree) {
     FreeAllTemporary();
 }
 
+SolveResult SolveSpaceUI::TestRankForGroup(hGroup hg) {
+    WriteEqSystemForGroup(hg);
+    Group *g = SK.GetGroup(hg);
+    SolveResult result = sys.SolveRank(g, NULL, NULL, false, false,
+                                       /*forceDofCheck=*/!g->dofCheckOk);
+    FreeAllTemporary();
+    return result;
+}
+
 bool SolveSpaceUI::ActiveGroupsOkay() {
     for(int i = 0; i < SK.groupOrder.n; i++) {
         Group *g = SK.GetGroup(SK.groupOrder.elem[i]);
diff --git a/src/solvespace.h b/src/solvespace.h
index f88b185..e650fb8 100644
--- a/src/solvespace.h
+++ b/src/solvespace.h
@@ -397,9 +397,15 @@ public:
 
     bool NewtonSolve(int tag);
 
+    void MarkParamsFree(bool findFree);
+    int CalculateDof();
+
     SolveResult Solve(Group *g, int *dof, List<hConstraint> *bad,
                       bool andFindBad, bool andFindFree, bool forceDofCheck = false);
 
+    SolveResult SolveRank(Group *g, int *dof, List<hConstraint> *bad,
+                          bool andFindBad, bool andFindFree, bool forceDofCheck = false);
+
     void Clear();
 };
 
@@ -847,6 +853,8 @@ public:
                      bool genForBBox = false);
     void SolveGroup(hGroup hg, bool andFindFree);
     void SolveGroupAndReport(hGroup hg, bool andFindFree);
+    SolveResult TestRankForGroup(hGroup hg);
+    void WriteEqSystemForGroup(hGroup hg);
     void MarkDraggedParams();
     void ForceReferences();
 
diff --git a/src/system.cpp b/src/system.cpp
index 5e6b807..e90611a 100644
--- a/src/system.cpp
+++ b/src/system.cpp
@@ -475,28 +475,8 @@ SolveResult System::Solve(Group *g, int *dof, List<hConstraint> *bad,
         // This is not the full Jacobian, but any substitutions or single-eq
         // solves removed one equation and one unknown, therefore no effect
         // on the number of DOF.
-        if(dof) *dof = mat.n - mat.m;
-
-        // If requested, find all the free (unbound) variables. This might be
-        // more than the number of degrees of freedom. Don't always do this,
-        // because the display would get annoying and it's slow.
-        for(i = 0; i < param.n; i++) {
-            Param *p = &(param.elem[i]);
-            p->free = false;
-
-            if(andFindFree) {
-                if(p->tag == 0) {
-                    p->tag = VAR_DOF_TEST;
-                    WriteJacobian(0);
-                    EvalJacobian();
-                    int rank = CalculateRank();
-                    if(rank == mat.m) {
-                        p->free = true;
-                    }
-                    p->tag = 0;
-                }
-            }
-        }
+        if(dof) *dof = CalculateDof();
+        MarkParamsFree(andFindFree);
     }
     // System solved correctly, so write the new values back in to the
     // main parameter table.
@@ -537,9 +517,71 @@ didnt_converge:
     return rankOk ? SolveResult::DIDNT_CONVERGE : SolveResult::REDUNDANT_DIDNT_CONVERGE;
 }
 
+SolveResult System::SolveRank(Group *g, int *dof, List<hConstraint> *bad,
+                              bool andFindBad, bool andFindFree, bool forceDofCheck)
+{
+    WriteEquationsExceptFor(Constraint::NO_CONSTRAINT, g);
+
+    // All params and equations are assigned to group zero.
+    param.ClearTags();
+    eq.ClearTags();
+
+    if(!forceDofCheck) {
+        SolveBySubstitution();
+    }
+
+    // Now write the Jacobian, and do a rank test; that
+    // tells us if the system is inconsistently constrained.
+    if(!WriteJacobian(0)) {
+        return SolveResult::TOO_MANY_UNKNOWNS;
+    }
+
+    bool rankOk = TestRank();
+    if(!rankOk) {
+        if(!g->allowRedundant) {
+            if(andFindBad) FindWhichToRemoveToFixJacobian(g, bad, forceDofCheck);
+        }
+    } else {
+        // This is not the full Jacobian, but any substitutions or single-eq
+        // solves removed one equation and one unknown, therefore no effect
+        // on the number of DOF.
+        if(dof) *dof = CalculateDof();
+        MarkParamsFree(andFindFree);
+    }
+    return rankOk ? SolveResult::OKAY : SolveResult::REDUNDANT_OKAY;
+}
+
 void System::Clear() {
     entity.Clear();
     param.Clear();
     eq.Clear();
     dragged.Clear();
 }
+
+void System::MarkParamsFree(bool find) {
+    // If requested, find all the free (unbound) variables. This might be
+    // more than the number of degrees of freedom. Don't always do this,
+    // because the display would get annoying and it's slow.
+    for(int i = 0; i < param.n; i++) {
+        Param *p = &(param.elem[i]);
+        p->free = false;
+
+        if(find) {
+            if(p->tag == 0) {
+                p->tag = VAR_DOF_TEST;
+                WriteJacobian(0);
+                EvalJacobian();
+                int rank = CalculateRank();
+                if(rank == mat.m) {
+                    p->free = true;
+                }
+                p->tag = 0;
+            }
+        }
+    }
+}
+
+int System::CalculateDof() {
+    return mat.n - mat.m;
+}
+