diff --git a/mlir/lib/Dialect/Affine/IR/AffineOps.cpp b/mlir/lib/Dialect/Affine/IR/AffineOps.cpp
--- a/mlir/lib/Dialect/Affine/IR/AffineOps.cpp
+++ b/mlir/lib/Dialect/Affine/IR/AffineOps.cpp
@@ -336,8 +336,11 @@
   // The dim op is also okay if its operand memref is a view/subview whose
   // corresponding size is a valid symbol.
   std::optional<int64_t> index = getConstantIntValue(dimOp.getDimension());
-  assert(index.has_value() &&
-         "expect only `dim` operations with a constant index");
+
+  // Be conservative if we can't understand the dimension.
+  if (!index.has_value())
+    return false;
+
   int64_t i = index.value();
   return TypeSwitch<Operation *, bool>(dimOp.getShapedValue().getDefiningOp())
       .Case<memref::ViewOp, memref::SubViewOp, memref::AllocOp>(
diff --git a/mlir/test/Dialect/Affine/invalid.mlir b/mlir/test/Dialect/Affine/invalid.mlir
--- a/mlir/test/Dialect/Affine/invalid.mlir
+++ b/mlir/test/Dialect/Affine/invalid.mlir
@@ -501,3 +501,17 @@
   affine.delinearize_index %idx into () : index
   return
 }
+
+// -----
+
+func.func @dynamic_dimension_index() {
+  "unknown.region"() ({
+    %idx = "unknown.test"() : () -> (index)
+    %memref = "unknown.test"() : () -> memref<?x?xf32>
+    %dim = memref.dim %memref, %idx : memref<?x?xf32>
+    // expected-error @below {{op index must be a dimension or symbol identifier}}
+    affine.load %memref[%dim, %dim] : memref<?x?xf32>
+    "unknown.terminator"() : () -> ()
+  }) : () -> ()
+  return
+}