diff --git a/mlir/lib/IR/Diagnostics.cpp b/mlir/lib/IR/Diagnostics.cpp
index f4c9242ed3479..5caf826c84bdd 100644
--- a/mlir/lib/IR/Diagnostics.cpp
+++ b/mlir/lib/IR/Diagnostics.cpp
@@ -600,9 +600,17 @@ struct ExpectedDiag {
   /// Emit an error at the location referenced by this diagnostic.
   LogicalResult emitError(raw_ostream &os, llvm::SourceMgr &mgr,
                           const Twine &msg) {
-    SMRange range(fileLoc, SMLoc::getFromPointer(fileLoc.getPointer() +
-                                                 substring.size()));
-    mgr.PrintMessage(os, fileLoc, llvm::SourceMgr::DK_Error, msg, range);
+    // fileLoc may be invalid when the expected diagnostic used an unknown
+    // location specifier (e.g. `// expected-error @unknown {{...}}`). In that
+    // case, skip the source range to avoid a null-pointer dereference and an
+    // assertion in SMRange that both endpoints must have the same validity.
+    if (fileLoc.isValid()) {
+      SMRange range(fileLoc, SMLoc::getFromPointer(fileLoc.getPointer() +
+                                                   substring.size()));
+      mgr.PrintMessage(os, fileLoc, llvm::SourceMgr::DK_Error, msg, range);
+    } else {
+      mgr.PrintMessage(os, fileLoc, llvm::SourceMgr::DK_Error, msg);
+    }
     return failure();
   }
 
diff --git a/mlir/test/mlir-opt/expected-unknown-loc-unmatched.mlir b/mlir/test/mlir-opt/expected-unknown-loc-unmatched.mlir
new file mode 100644
index 0000000000000..5a06d386dabf8
--- /dev/null
+++ b/mlir/test/mlir-opt/expected-unknown-loc-unmatched.mlir
@@ -0,0 +1,9 @@
+// Test that an unmatched `expected-*` directive using the `@unknown` location
+// specifier emits a diagnostic instead of crashing.
+// See https://github.com/llvm/llvm-project/issues/163343
+
+// RUN: not mlir-opt --verify-diagnostics %s 2>&1 | FileCheck %s
+
+// CHECK: expected warning "some warning that is never produced" was not produced
+
+// expected-warning @unknown {{some warning that is never produced}}