diff --git a/mlir/test/Bytecode/bytecode_callback.mlir b/mlir/test/Bytecode/bytecode_callback.mlir
index 3b537bb974da..a3f6883f5323 100644
--- a/mlir/test/Bytecode/bytecode_callback.mlir
+++ b/mlir/test/Bytecode/bytecode_callback.mlir
@@ -1,5 +1,6 @@
 // RUN: mlir-opt %s --test-bytecode-roundtrip="test-dialect-version=1.2" -verify-diagnostics | FileCheck %s --check-prefix=VERSION_1_2
 // RUN: mlir-opt %s --test-bytecode-roundtrip="test-dialect-version=2.0" -verify-diagnostics | FileCheck %s --check-prefix=VERSION_2_0
+// RUN: mlir-opt %s --split-input-file --test-bytecode-roundtrip="test-dialect-version=2.0" -verify-diagnostics | FileCheck %s --check-prefix=NO_TEST_DIALECT
 
 func.func @base_test(%arg0 : i32) -> f32 {
   %0 = "test.addi"(%arg0, %arg0) : (i32, i32) -> i32
@@ -12,3 +13,15 @@ func.func @base_test(%arg0 : i32) -> f32 {
 
 // VERSION_2_0-NOT: Overriding IntegerType encoding...
 // VERSION_2_0-NOT: Overriding parsing of IntegerType encoding...
+
+// -----
+
+// Regression test for https://github.com/llvm/llvm-project/issues/128321:
+// test-bytecode-roundtrip must not crash when the test dialect is absent from
+// the module (i.e., no test dialect types are present in the bytecode).
+
+// NO_TEST_DIALECT-NOT: Overriding IntegerType encoding...
+// NO_TEST_DIALECT: func.func @no_test_dialect_ops
+func.func @no_test_dialect_ops() {
+  return
+}
diff --git a/mlir/test/lib/IR/TestBytecodeRoundtrip.cpp b/mlir/test/lib/IR/TestBytecodeRoundtrip.cpp
index e83a3432a028..d7e224f8b04e 100644
--- a/mlir/test/lib/IR/TestBytecodeRoundtrip.cpp
+++ b/mlir/test/lib/IR/TestBytecodeRoundtrip.cpp
@@ -166,10 +166,12 @@ private:
     parseConfig.getBytecodeReaderConfig().attachTypeCallback(
         [&](DialectBytecodeReader &reader, StringRef dialectName,
             Type &entry) -> LogicalResult {
-          // Get test dialect version from the version map.
+          // Get test dialect version from the version map. If the test dialect
+          // is not present in the bytecode (e.g., the module contains no test
+          // dialect types), version info will be absent -- just skip.
           auto versionOr = reader.getDialectVersion<test::TestDialect>();
-          assert(succeeded(versionOr) && "expected reader to be able to access "
-                                         "the version for test dialect");
+          if (failed(versionOr))
+            return success();
           const auto *version =
               reinterpret_cast<const test::TestDialectVersion *>(*versionOr);
           if (version->major_ >= 2)