diff --git a/mlir/lib/Analysis/Presburger/PresburgerSet.cpp b/mlir/lib/Analysis/Presburger/PresburgerSet.cpp
--- a/mlir/lib/Analysis/Presburger/PresburgerSet.cpp
+++ b/mlir/lib/Analysis/Presburger/PresburgerSet.cpp
@@ -409,15 +409,29 @@
   assert(i != j && "The indices must refer to different polyhedra");
 
   unsigned n = polyhedrons.size();
-  polyhedrons[i] = polyhedrons[n - 1];
-  polyhedrons[j] = polyhedrons[n - 2];
-  polyhedrons.pop_back();
-  polyhedrons[n - 2] = poly;
-
-  simplices[i] = simplices[n - 1];
-  simplices[j] = simplices[n - 2];
-  simplices.pop_back();
-  simplices[n - 2] = Simplex(poly);
+  // This case needs special handling since position `n` - 1 is removed from the
+  // vector, hence the `IntegerPolyhedron` at position `n` - 2 is lost
+  // otherwise.
+  if (j == n - 1) {
+    polyhedrons[i] = polyhedrons[n - 2];
+    polyhedrons.pop_back();
+    polyhedrons[n - 2] = poly;
+
+    simplices[i] = simplices[n - 2];
+    simplices.pop_back();
+    simplices[n - 2] = Simplex(poly);
+
+  } else {
+    polyhedrons[i] = polyhedrons[n - 1];
+    polyhedrons[j] = polyhedrons[n - 2];
+    polyhedrons.pop_back();
+    polyhedrons[n - 2] = poly;
+
+    simplices[i] = simplices[n - 1];
+    simplices[j] = simplices[n - 2];
+    simplices.pop_back();
+    simplices[n - 2] = Simplex(poly);
+  }
 }
 
 /// Given two polyhedra `a` and `b` at positions `i` and `j` in `polyhedrons`
diff --git a/mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp b/mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp
--- a/mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp
+++ b/mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp
@@ -645,6 +645,17 @@
   expectCoalesce(3, set);
 }
 
+TEST(SetTest, coalesceLastCoalesced) {
+  PresburgerSet set = parsePresburgerSetFromPolyStrings(
+      1, {
+             "(x) : (x == 0)",
+             "(x) : (x - 1 >= 0, -x + 3 >= 0)",
+             "(x) : (x + 2 == 0)",
+             "(x) : (x - 2 >= 0, -x + 4 >= 0)",
+         });
+  expectCoalesce(3, set);
+}
+
 TEST(SetTest, coalesceDiv) {
   PresburgerSet set =
       parsePresburgerSetFromPolyStrings(1, {