diff --git a/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.h b/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.h
--- a/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.h
+++ b/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.h
@@ -220,8 +220,10 @@
   void setElideExpr(bool b) { elideExpr = b; }
   constexpr SparseTensorDimSliceAttr getSlice() const { return slice; }
 
-  /// Checks whether the variables bound/used by this spec are valid
-  /// with respect to the given ranks.
+  /// Checks whether the variables bound/used by this spec are valid with
+  /// respect to the given ranks.  Note that null `DimExpr` is considered
+  /// to be vacuously valid, and therefore calling `setExpr` invalidates
+  /// the result of this predicate.
   bool isValid(Ranks const &ranks) const;
 
   // TODO(wrengr): Use it or loose it.
diff --git a/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.cpp b/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.cpp
--- a/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.cpp
+++ b/mlir/lib/Dialect/SparseTensor/IR/Detail/DimLvlMap.cpp
@@ -150,8 +150,6 @@
     rhs.printStrong(os);
   } else {
     // Combination of all the special rules for addition/subtraction.
-    // TODO(wrengr): despite being succinct, this is prolly too confusing for
-    // readers.
     lhs.printWeak(os);
     const auto rx = matchNeg(rhs);
     os << (rx ? " - " : " + ");
@@ -180,8 +178,9 @@
     : var(var), expr(expr), slice(slice) {}
 
 bool DimSpec::isValid(Ranks const &ranks) const {
-  return ranks.isValid(var) && ranks.isValid(expr);
-  // TODO(wrengr): is there anything in `slice` that needs validation?
+  // Nothing in `slice` needs additional validation.
+  // We explicitly consider null-expr to be vacuously valid.
+  return ranks.isValid(var) && (!expr || ranks.isValid(expr));
 }
 
 bool DimSpec::isFunctionOf(VarSet const &vars) const {
@@ -220,8 +219,8 @@
 }
 
 bool LvlSpec::isValid(Ranks const &ranks) const {
+  // Nothing in `type` needs additional validation.
   return ranks.isValid(var) && ranks.isValid(expr);
-  // TODO(wrengr): is there anything in `type` that needs validation?
 }
 
 bool LvlSpec::isFunctionOf(VarSet const &vars) const {
diff --git a/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h b/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h
--- a/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h
+++ b/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h
@@ -179,7 +179,10 @@
 public:
   constexpr Var(VarKind vk, Num n) : impl(Impl(vk, n)) {}
   Var(AffineSymbolExpr sym) : Var(VarKind::Symbol, sym.getPosition()) {}
-  Var(VarKind vk, AffineDimExpr var) : Var(vk, var.getPosition()) {}
+  // TODO(wrengr): Should make the first argument an `ExprKind` instead...?
+  Var(VarKind vk, AffineDimExpr var) : Var(vk, var.getPosition()) {
+    assert(vk != VarKind::Symbol);
+  }
 
   constexpr bool operator==(Var other) const { return impl == other.impl; }
   constexpr bool operator!=(Var other) const { return !(*this == other); }
@@ -345,13 +348,8 @@
   enum class ID : unsigned {};
 
 private:
-  // FUTURE_CL(wrengr): We could use the high-bit of `Var::Impl` to
-  // store the `std::optional` bit, therefore allowing us to bitbash the
-  // `num` and `kind` fields together.
-  //
   StringRef name;              // The bare-id used in the MLIR source.
   llvm::SMLoc loc;             // The location of the first occurence.
-                               // TODO(wrengr): See the above `LocatedVar` note.
   ID id;                       // The unique `VarInfo`-identifier.
   std::optional<Var::Num> num; // The unique `Var`-identifier (if resolved).
   VarKind kind;                // The kind of variable.
diff --git a/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp b/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp
--- a/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp
+++ b/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp
@@ -33,10 +33,9 @@
 //===----------------------------------------------------------------------===//
 
 bool Ranks::isValid(DimLvlExpr expr) const {
-  // FIXME(wrengr): we have cases without affine expr at an early point
-  if (!expr.getAffineExpr())
-    return true;
-  // Each `DimLvlExpr` only allows one kind of non-symbol variable.
+  assert(expr);
+  // Compute the maximum identifiers for symbol-vars and dim/lvl-vars
+  // (each `DimLvlExpr` only allows one kind of non-symbol variable).
   int64_t maxSym = -1, maxVar = -1;
   // TODO(wrengr): If we run into ASan issues, that may be due to the
   // "`{{...}}`" syntax; so we may want to try using local-variables instead.